INIA GEOFFRENSIS AND PONTOPORIA BLAINVILLII. 111 
2. Opontocetr or DeupHinorpEa. Teeth always developed after birth, and generally 
numerous, sometimes few and early deciduous. No baleen. Olfactory organ rudi- 
mentary or absent. External respiratory aperture single. Upper surface of the 
skull generally, if not always, unsymmetrical. Hinder end of the maxilla ex- 
panded, and covering the greater part of the orbital plate of the frontal bone. 
Lachrymal bone either inseparable from the jugal or, when distinct, very large 
and forming part of the roof of the orbit. Rami of mandible nearly straight, 
much expanded in height posteriorly, and coming into contact in front by a surface 
of variable length, but always constituting a true symphysis. Sternum almost 
always composed of several pieces placed one behind the other, and always con- 
nected with several pairs of ribs, either by cartilage or by distinct costo-sternal 
bones. Many of the ribs with capitular processes developed, and articulating 
with the bodies of the vertebre. 
It is not necessary to pursue further the arrangement of the M/ystacoceti, as it has no 
direct bearing upon the subject of this memoir, and as moreover I have no reason to 
make any alteration in the divisions into families and genera sketched out in the paper 
above referred to. 
The subdivision of the Odontoceti, according to their structural affinities, presents at 
first sight considerable difficulty. To relate all the various attempts, more or less 
successful, that have been made to unravel this problem would be out of place here. 
I will only add one more to the number, founded chiefly on an examination of the 
osteological characters of the principal members of the group*. 
In seeking for some starting-point from which to commence the formation of a 
natural division of the Toothed Whales, one has occurred to me which I have not found 
hitherto noticed. The strong and well-defined bones which connect the ribs with the 
sternum, ossified even at birth, common to the Porpoise, true Dolphins, and their 
nearest allies, are represented even in the adult Hyperoodon by an entirely unossified 
cartilage. In the four skeletons of Physeter macrocephalus that I have had the oppor- 
tunity of examining, I have looked in vain for sterno-costal bones, some of which would 
certainly have been preserved if they approached in relative magnitude and density 
those of the true Dolphins. In answer to my inquiries on the subject, Dr. George 
Bennett has kindly informed me that, in both the skeletons of the genus Kogia, now 
mounted in the Sydney Museum, the cartilages are unossified; and I am indebted to 
Professor Van Beneden for similar information respecting the skeleton of the ziphioid 
Micropteron preserved in the Zoological Museum at Brussels. From these facts, I think 
that we may safely infer that the absence of ossified sternal ribs is a character common 
to the large natural group which includes Physeter, Hyperoodon, and the Ziphioids. To 
© The arrangement here proposed nearly coincides with that arrived at by Professor Huxley and myself, 
when discussing this subject together before the delivery of the course of Hunterian Lectures at the Royal 
College of Surgeons for the present year (see ‘ Lancet,’ 1866, vol. i. p. 381). 
VoL. VI.—PART III. R 
