342 FOSSILS FROM THE 



stages of the foetal state, continues to manifest the usual mam- 

 malian character, by presenting a strongly indicated suture at 

 the symphysis, the mark of which can be traced, in at least 

 the interior of the jaw, throughout life. Now, in the first 

 ganoids, the under jaw usually consisted, as in the j)lacoids 

 and most of the mammalia, of two bones, — one on each side. 

 Such was its character in the genera Aster olepis, Glyptolepis, 

 Osteolepis, Diplopterus, and Coccosteus. There was, however 

 at least one of the ganoidal contemporaries of these fishes, — 

 the Dipterus [Spec. 1 6], — in which the under jaw formed, as in 

 the human subject, a single bone. In the foetal state it consist- 

 ed apparently of three bones, one on each side, and a central 

 or keybone ; at least a line of foramina seems to indicate that 

 on eitlier side there had once existed a suture ; but in all 

 the more perfect specimens of Dipteri yet found, the lower 

 jaw, as a whole, is but one bone. In general form, though 

 not in this circumstance of unity, it resembled, seen from be- 

 low, the under jaw of some of the grampus family ; and it 

 must altogether, especially in the well-defined character of its 

 condyloid and coronoid processes, have been more like the jaw 

 ofa mammal than that of a fish. [Spec. 17.] In most of the or- 

 dinary fish&s, the hinging of the jaw, if I may so speak, is part- 

 ly osseous, partly cartilaginous : in these, too, and in the pla- 

 coids, what may be termed the male half of the hinge belongs 

 to the head, and the female half to the jaw. In the mam- 

 malia, on the contrary, the hinge is altogether formed of solid 

 bone, and the male and female halves of the hinge change 

 places, — the male half being that of the jaw, and the female 

 half scooped out of the head. This is strikingly the case in 

 the head of the badger, in which the joint is so complete, and 

 the cranial socket so thoroughly envelopes the condyloid ball, 

 that without fracture dislocation of the jaw is impossible. It 

 is also very complete, though less so in the head of the hyena : 

 and there are indeed few mammals that do not present an 



