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taking up more than half the whole dorsal face of the radial, while in the younger specimen it occupies much less than half.

The effect of this change in the component parts of the radial pentagon is to give its central synosteal surface a considerable slope inwards and downwards, so that the whole, when viewed from above, has the form of a wide and shallow funnel. The rim of the funnel (outer dorsal surfaces of the radials) is thick in the young specimen, but does not increase with the growth of the interior (inner dorsal surfaces). Consequently, the centrodorsal which forms, as it were, a plug fitting into the funnel, slips farther and farther down into it, until its dorsal surface becomes level with that of the radial pentagon, or even comes to be actually below it. At the same time it loses its few marginal cirrhi, and their sockets become obliterated, so that the whole dorsal surface of the calyx is one uniform plane. Act. Jukesii ri mains permanently in this condition; but there are other species, as we have seen, and notably Act. stellata, in which the centrodorsal loses its pentagonal shape, owing to the appearance of more or less deep clefts between its outer edge and the inner edges of the radials.

In Act. pectinata the ventral face of the centrodorsal is divided by ridges into five radial areas, corresponding with the five synosteal surfaces of the first radials that rest upon it. These radial areas are occupied by median depressions, which increase somewhat in depth from their peripheral to their central ends. But the synosteal surfaces of the radials do not exhibit corresponding ridges, for they are marked by similar median depressions, which are also deepest at their central ends. When, therefore, the synosteal surface of the radial pentagon and the ventral surface of the centrodorsal are in their normal state of apposition, they are separated from one another along the median lines of the five radials by five cavities or

radial spaces.” Those are largest at their blind central ends, and extend in a peripheral direction to open externally by five minute openings, situated round the margin of the small centrodorsal piece, beneath the radial pentagon which rests upon it, and extends considerably beyond it. It seems to me that we have here an explanation of the large openings between the radials and centrodorsal of Act. stellata and Phano. genia, &c.

In Act. pectinata these radial spaces end blindly around the central cavity of the radial pentagon, being shut off from it by the thickened inner margin of its synosteal surface. Whether they are also blind in Act. stellata, in which they are so very large, or whether they are in communication with the radial diverticula of the colom, which are inclosed within the spouts of the rosette, is a point which can only be settled by making a series of sections through the decalcified calyx.

I have elsewhere (Actinometra, cap. iv, § 61) drawn attention to the homology of these openings between the radial pentagon and

VOL. XXVIII.

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upon them.

394 On the Comatulæ of the ChallengerExpedition. [Mar. 6, centrodorsal of Act. pectinata with the openings on the outside of the calyx of Apiocrinus rotundus and Ap. obconicus, which are situated between every pair of continuous basals, and the radials which rest

Other homologues are the radially situated “interarticular pores” in the upper part of the stem of Pentacrinus.

It is worth notice, that all the species in which the centrodorsal exhibits these variations of form are true Actinometræ, i.e., they have an eccentric mouth and a terminal comb on the oral pinnules. In Lovèn's Phanogenia, however, the mouth is central, and there is a terminal comb to the oral pinnales. It is thus a very singular exception, for I know of no Antedon in which the oral pinnules have this terminal comb, nor one in which the centrodorsal has anything like the form which it has in Phanogenia.

In fact, I am able to say that the examination of the “ Challenger” Comatulæ has entirely confirmed the opinions held by Dr. Lütken and myself (Actinometra, cap. ii, $$ 14, 15) respecting the distinguishing characters of Antedon and Actinometra. We both agree in referring forms with a (sub) central mouth, five equal ambulacra, and no terminal comb on the oral pinnules, to Antedon. On the other hand, species with an eccentric mouth, a variable number of unequal ambulacra, and a terminal comb to the oral pinnules, belong to Actinometra. There are only two specimens in the “Challenger" collection which have an eccentric mouth but no terminal comb. Pourtales' Comatula meridionalis appears to be another, but these are only three exceptions out of some sixty species.

It will be seen at once that these characters are of no use in distinguishing the genera of fossil Comatulæ. But, as has been hinted above, there are very considerable differences in the shape of the radials and centrodorsal piece in Antedon and Actinometra respectively, and as these are exactly the parts which are most met with as fossils, the generic determination of a fossil form is almost as easy as that of a recent one, which has given up its disk to produce a Hyponome. As I have described these differences very fully in my

Actinometra memoir (cap. iv, § 41, 51, 54-56), it is not necessary to do more than refer to them here, with the remark that a more extended knowledge of the species of both genera has only strengthened the opinions which I have there expressed.

The same is the case with regard to the so-called “ventral nerve of Comatula, viz., the fibrillar band underlying the epithelium of the ambulacral grooves.

I have already shown (Actinometra, cap. iii, $ 23-26) that, in Act. polymorpha and Act. solaris, half, or even more than half, of the arms may have neither groove, epithelium,“ nerve," nor tentacles, and I have insisted, as strongly as possible, on the important bearing of this fact on the Ludwig-Gegenbaur view that these subepithelial bands constitute the nervous system of the Crinoids. Neither of these two authors has referred to my statements at all, but both have entirely ignored them. I am now able to repeat them, and to give them much greater force. No less than twenty-three out of the forty-eight species of " Challenger" Actinometra, and three species in Semper's collection, have more or fewer grooveless arms. I have cut sections of these arms in two species, and have obtained the same results as with Act. polymorpha and Act. solaris. The “ventral nerve" and ambulacral epithelium are conspicuous by their absence, while the axial cords in the skeleton, which I also regard as true nerves, give off branches freely in the centre of each arm-joint, as I have already described for other species both of Actinometra and of Antedon. Two points are noteworthy. In one species, one of the posterior ambulacral grooves stops quite abruptly on the disk, some little

way from the arm bases, and the two arms to which it would naturally have gone with its "nerve," tentacles, &c., receive no branches from any of the adjacent grooves to supply the deficiency.

Lastly, in the gigantic Philippine species already referred to as No. 37, there are more than one hundred arms, many of which are grooveless and "nerveless," as I have found by section-cutting. But these abnormal arms are not limited to the posterior part of the body, as is usually the case, for there are several on each radius.

Evidence of this negative character appears to me to be a serious objection to the German view that the subepithelial bands constitute the only nervous apparatus of the Crinoids. Ludwig* attacks Lange's opinions as to the Asterid-nerves, on the ground that the structures supposed by Lange to be nerves are not constant, but are absent from the arms of certain species. It is curious, however, that Ludwig is unable to apply this reasoning to his own views respecting the nerves of the Crinoids !

III. “On the Characters of the Pelvis in the Mammalia, and the

Conclusions respecting the Origin of Mammals which may be based on them.” By Professor HUXLEY, Sec. R.S., Professor of Natural History in the Royal School of Mines. Received February 24, 1879.

[PLATE 8.] In the course of the following observations upon

the typical characters and the modifications of the pelvis in the Mammalia, it will be convenient to refer to certain straight lines, which may be drawn through anatomically definable regions of the pelvis, as arer.

"Beiträge zur Anatomie der Asteriden." Zeitschr. für Wiss. Zool.," Band XXI, p. 191.

66

Of these I shall term a longitudinal line traversing the centre of the sacral vertebræ, the sacral axis (Plate 8, S.a.); a second, drawn along the ilium, dorso-ventrally, through the middle of the sacral articulation and the centre of the acetabulum, will be termed the iliac axis (11. a.); a third, passing through the junctions of the pubis and ischium above and below the obturator foramen, will be the obturator axis (Ob. a.); while a fourth, traversing the union of the ilium, in front with the pubis, and behind with the ischium, will be the iliopectineal axis (Ip. a.).

The least modified form of mammalian pelvis is to be seen, as might be expected, in the Monotremes, but there is a great difference between Ornithorhynchus and Echidna in this respect, the former being much less characteristically mammalian than the latter.

In Ornithorhynchus (Plate 8, fig. 4), the ilium is remarkably narrow, and the angle between the iliac and the sacral axis is large, so that the ilium is but very slightly inclined backwards. The iliopectineal axis, nearly at right angles with the iliac axis, is inclined to the sacral axis at an acute angle; while the obturator axis is nearly perpendicular to the sacral axis, and the obturator foramen is relatively small. The front margin of the cotyloid end of the pelvis sends off a very strong pectineal process (p. p.), from the inferior basal part of which a short, obtnse tuberculum pubis (t. p.) projects. Between this and the symphysis, the base of the marsupial bone (Ep. p.) is attached. The ventral rami of the pubes are short and, like those of the ischium, they are united throughout their whole length in a long symphysis, the ischial division of which (Sy. I.) is as long as, if not longer than, the pubic division (Sy.p.). The cotyloid ramus of each ischium gives off a stout elongated metischial process (m. p.) backwards.

In Echidna (Plate 8, fig. 5), on the other hand, the ilium is much broader; while the iliac axis inclines downwards and backwards, at an acute angle with the sacral axis. The iliopectineal axis being still at right angles with the iliac axis, makes a much larger angle with the sacral axis; and the obturator axis is inclined from above, at an angle of nearly 45° to the sacral axis, downwards and backwards. In fact, the change in the general character of the pelvis seems to result from its ventral elements having been carried backwards and upwards by backward and upward shifting of that portion of the ilinm which lies below the level of its articulation with the sacrum. There are other changes by which the aspect of the pelvis is much altered. The inner wall of the acetabulum is incompletely ossified, but, in other respects, the pelvis makes a considerable approximation towards the ordinary mammalian form. Thus the pectineal process is represented by a less prominent and more elongated ridge; the metischial process widens out into a mere triangular expansion or "tuberosity," of the ischium, and the symphysial union of the ischia is short.

In all other Mammalia (e.g., Lepus, Plate 8, fig. 6) the iliac axis forms as acute, if not a more acute, angle with the sacral axis ; the angle between the ilio pectineal axis and the sacral axis more and more approaches a right angle; and that between the sacral axis and the obturator axis becomes more and more acute. The obturator foramen acquires a much larger proportional size. The symphysial union becomes restricted to a greater or less portion of the pubes ; or the veutral halves of the ossa innominata may cease be directly united, even the pubes being far apart in the dry skeleton. The metischial processes are represented by tuberosities, which may extend upwards and unite with anterior caudal vertebræ ; and the ilia may remain narrow or become extremely expanded. In all monodelphous Mammalia the marsupial bones disappear.

The distinctive features of the mammalian pelvis have been clearly indicated by Gegenbaur, * who points out that in mammals, in contradistinction from reptiles, “the longitudinal axis of the ilium gradually acquires an oblique direction, from in front and above, backwards and downwards. The part which represents the crista above thus becomes turned forwards, or more or less outwards, with increase of lateral surface; the acetabular part backwards and downwards; hence the ischium retains its original direction in the produced long axis of the ilium and, at the same time, takes up a position in relation to the vertebral column similar to that which obtains in birds. The conditions of this position are, however, to be sought in factors of a totally different nature in mammals from those which produce it in birds; for, in the former, the ischium follows the changed direction of the ilium, wbilst in birds, the ilium has nothing to do with the matter, and the ventral elements of the pelvis appear to pass towards the caudal region, independently of the ilium.”

On one point, however, I cannot agree with Gegenbaur's conclusions. He is of opinion that the ilium of mammals answers to the post-acetabular part of the ilium of birds, and that "the crista 088is ilii of mammals corresponds with the posterior edge of the postacetabular part of the bird's ilium. Between the two parts, therefore, there is the difference of a rotation through an angle of almost 180°.” On the contrary, it appears to me evident that the whole crista ilii in a mammal corresponds with the whole dorsal edge of the ilium in a bird or a reptile, and that the angle through which the iliac axis rotates amounts to not more than 90° (compare Plate 8, fig. 6, Lepus, with fig. 9, Apteryx). I cannot reconcile the contrary view either with the relations of the ilium to the sacrum, or with the attachment of the muscles.

On comparing the pelvis of Ornithorhynchus with that of a lizard

· Beiträge zur Kenntniss des Beckens der Vögel," “ Jenaische Zeitschrift," vi.

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