Unless the like should prove to be the case in the Dicynodonts, it is in the Mammalia alone that the subsacral portion of the ilium elongates backwards, carrying with it the pubis and the ischium, between which a large rounded obturator fontanelle is developed.

These facts appear to me to point to the conclusion that the Mammalia have been connected with the Amphibia by some unknown" promammalian" group, and not by any of the known forms of Sauropsida ; and there is other evidence which tends in the same direction.

Thus, the Amphibia are the only air-breathing Vertebrata which, like Mammals, have a dicondylian skull. It is only in them that the articular element of the mandibular arch remains cartilaginous; while the quadrate ossification is small, and the squamosal extends down over it to the osseous elements of the mandible; thus affording an easy transition to the mammalian condition of these parts.

The pectoral arch of the Monotremes is as much amphibian as it is sauropsidan; the carpus and the tarsus of all Sauropsida, except the Chelonia, are modified away from the Urodele type, while those of the Mammal are directly reducible to it; and it is perhaps worth notice, that the calcar of the frogs is, in some respects, comparable with the spur of the Monotremes.

Finally, the fact that, in all Sauropsida, it is a right aortic arch which is the main conduit of arterial blood leaving the heart, while, in Mammals, it is a left aortic arch which performs this office, is a great stumbling-block in the way of the derivation of the Mammalia from any of the Sauropsida. But, if we suppose the earliest forms of both the Mammalia and the Sauropsida to have had a common Amphibian origin, there is no difficulty in the supposition that, from the first, it was a left aortic arch in the one series, and the corresponding right aortic arch in the other, which became the predominant feeder of the arterial system.

The discovery of the intermediate links between Reptilia and Aves, among extinct forms of life, gives every ground for hoping that, before long, the transition between the lowest Mammalia at present known and the simpler Vertebrata may be similarly traced. The preceding remarks are intended to direct attention to the indications of the characters of these promammalian Vertebrata, which the evidence at present forthcoming seems to me to suggest.

In the relatively large size of the brain, and in the absence of teeth, the only existing representatives of the Ornithodelphia present characters which suggest that they are much modified members of the group. On comparing the brain of Echidna, for example, with that of many Marsupialia and Insectivora, its relative magnitude is remarkable: and, in view of the evidence which is now accumulating, that the brain increases in size in the later members of the same series of Mammalia, one may surmise that Echidna is the last

term of a series of smaller-brained Ornithodelphia. Among the higher Vertebrata, I think that there is strong reason to believe that edentulous animals are always modifications of toothed forms.

EXPLANATION OF PLATE 8.

Figs. 1 to 9. The left half of the pubic arch in Salamandra (fig. 1), Iguana (fig. 2), Crocodilus (fig. 3), Ornithorhynchus (fig. 4), Echidna (fig. 5), Lepus (fig. 6), Compsognathus (fig. 7), Laosaurus (fig. 8), and Apteryx (fig. 9). The letters have the same signification throughout. Il. ilium, Pb. pubis, Is. ischium, Ep.p. epipubis, S. a. sacral axis, Ip. a. ilio-pectineal axis, Ob. a. obturator axis, Il. a. iliac axis, Sy. p., Sy. I., indicate the extent to which the pubes and the ischia unite respectively in their ventral symphyses; p. p. pectineal process, t. p. tuberosity or spine of the pubis, m. p. metischial process or tuberosity of the ischium, Cl. os cloaca in Iguana.

Figs. 10, 11. Dorsal and ventral views of the ventral half of the pelvis of Salamandra

maculosa (×4). The letters as before, except Epp. Epp.2, right and left rami of the epipubis or ypsiloid cartilage, Ob. n. foramen of the obturator nerve, s. trace of a suture between the ischium and the pubis on the right side, Ac. acetabulum.

Fig. 12. Ventral aspect of the pelvis of a small Crocodilus acutus and the hindermost abdominal false ribs, of the natural size. The obturator nerve (Ob. n.) perforates the aponeurosis, which fills up the rhomboidal space between the ischia and the pubes, on the inner side of the pubis. Sy. p. l. the ligamentous union of the pubes.

Fig. 13. Dorsal aspect of the epipubis and the muscles connected with it in Salamandra maculosa. On the left side, the transversalis and the muscle C. are removed.

Fig. 15. The same, all the muscles but the external oblique being removed.
Fig. 14. The epipubis and the adjacent muscles in Ornithorhynchus.

R. rectus abdominus, Py. pyramidalis, O. e. obliquus externus, Trs. transversus, 1. a. linea alba, f. fascia, extending between the two rami of the epipubis in Salamandra, P.l. the representative of Poupart's ligament, i. r. inguinal ring, A. B. C. muscles of Salamandra described in the text. 4. and C. together appear to be the pubio-marsupial of Dugès: B. is the hebosteoglossus of von Siebold.

March 13, 1879.

THE PRESIDENT in the Chair.

The Presents received were laid on the table, and thanks ordered for them.

I. "The Influence of Electricity on Colliding Water Drops." By Lo RAYLEIGH, F.R.S. Received February 27, 1879.

It has been known for many years that electricity has an extraordinary influence upon the behaviour of fine jets of water ascending in a nearly vertical direction. In its normal state a jet resolves itself into drops, which even before passing the summit, and still more after passing it, are scattered through a considerable width. When a feebly electrified body is brought into its neighbourhood, the jet undergoes a remarkable transformation, and appears to become coherent; but under more powerful electrical action the scattering becomes even greater than at first. The second effect is readily attributed to the mutual repulsion of the electrified drops, but the action of feeble electricity in producing apparent coherence has been a mystery hitherto.

It has been shown by Beetz that the coherence is apparent only, and that the place where the jet breaks into drops is not perceptibly shifted by the electricity. By screening various parts with metallic plates, Beetz further proved that, contrary to the opinion of earlier observers, the seat of sensitiveness is not at the root of the jet where it leaves the orifice, but at the place of resolution into drops. As in Sir W. Thomson's water-dropping apparatus for atmospheric electricity, the drops carry away with them an electric charge, which may be collected by receiving the water in an insulated vessel.

I have lately succeeded in proving that the normal scattering of a nearly vertical jet is due to the rebound of the drops when they come into collision with one another. Such collisions are inevitable in consequence of the different velocities acquired by the drops under the action of the capillary force, as they break away irregularly from the continuous portion of the jet. Even when the resolution is regularised by the action of external vibrations of suitable frequency, as in the beautiful experiments of Savart and Plateau, the drops must still come into contact before they reach the summit of their parabolic path. In the case of a continuous jet the "equation of continuity" shows that as the jet loses velocity in ascending, it must increase in section. When the stream consists of drops following the same path in single file, no such increase of section is possible, and then the constancy of the total stream requires a gradual approximation of the drops, which in the case of a nearly vertical direction of motion cannot stop short of actual contact. Regular vibration has, however, the effect of postponing the collisions and consequent scattering of the drops, and in the case of a direction of motion less nearly vertical may prevent them altogether.