PROF. MARSH ON MAMMALS. 73 



The quadrate bone, always present in Birds, Reptiles, and the 

 other lower vertebrates, is well known as the suspensorium of the 

 lower jaw, which meets it with a concave articular face. The 

 quadrate, however, appears to be wanting in mammals, or at least 

 has not yet been identified with certainty. 



What represents the quadrate bone in mammals is a vexed 

 question in itself, and some of the best anatomists in the past, 

 Cuvier, Owen, Peters, Huxley, and others, have endeavoured to 

 solve the problem. The tympanic bone, the incus, and the malleus 

 have each in turn been regarded as the remnant of the quadrate, 

 but up to the present time the question has not been settled. It is 

 not improbable that the quadrate may have coalesced with the 

 squamosal. 



The occipital condyles of mammals, as well known, are two in 

 number, and separated from each other. This is not the case with 

 any true reptile, although the contrary has been asserted. The 

 nearest approach appears to be where there is a single bifid condyle, 

 cordate in shape, with the two lobes meeting below, as in some 

 reptiles and a few birds, but not separate as in mammals and 

 amphibians. 



Finally, in all known mammals, recent and extinct, the lower 

 jaw is composed of a single piece, and has a convex condyle meeting 

 the skull by a distinct articulation. All reptiles, even those supposed 

 to be nearest the mammals, have the lower jaw composed of several 

 pieces, and these show distinct sutures between them, a profound 

 difference that must be explained away before an approach can be 

 made between the two classes. 



It may fairly be said that the separate elements of the lower 

 jaw, if present, would naturally be looked for in the Mesozoic 

 mammals, and this point I have long had in mind. I may safely 

 say that I have seen nearly every species of Mesozoic mammals 

 hitherto described, and have searched for evidence on this point 

 without success. I have also sought for the separate elements in 

 the young of recent forms, but without finding any indications of 

 them. 



Beside the crucial points I have mentioned in the skull, there 

 are others of equal importance in the skeleton, which I must not 

 take time to discuss, but will venture to allude to one of them 

 in passing. I refer to the ankle joint, which, when present, is at 

 the end of the tibia in mammals, but in reptiles between the first 

 and second series of tarsals. When we really find an approach 

 between these two classes, the ankle joint will probably show 

 evidence of it. 



Having thus shown, as I believe, that we cannot with our 

 present knowledge expect to find the origin of mammals among 

 the known extinct reptiles, and that in attempting this we are 

 probably off the true line of descent, it remains to indicate another 

 direction in which the quest seems more promising. 



