14 Professor 0. C. Marsh — Origin of Mammals. 



In consideriiig the relations of reptiles to mammals so far as the 

 fossil forms are concerned, which seems to be the main question 

 now before us, I have only time to speak of the skull, and shall 

 refer to some of its salient features already mentioned in this dis- 

 cussion ; namely, the teeth, the squamosal bone, the quadrate, the 

 occipital condyles, and with them the lower jaw. These perhaps 

 may serve as crucial points in distinguishing the skull of a reptile 

 from that of a mammal, even if they fail to indicate a near affinity 

 between them. 



The different kinds of teeth seen in the reptiles regarded as 

 mammalian in type I consider of comparatively small importance, 

 for the reason that the same general forms of teeth are to be found, 

 not merely in the reptiles supposed to be nearest to mammals, but 

 also in other groups widely different. In the Crocodiles, for 

 example, the extinct genus Notosuchus, recently discovered in 

 Patagonia, has all three kinds of teeth well distinguished. Again, 

 some of the Dinosaurs, especially the genus Triceratops, have teeth 

 with two roots, a supposed mammalian character. In some Fishes, 

 also (Anarrhichas), three kinds of teeth may be seen. It is more 

 than probable, therefore, that the peculiar resemblance between the 

 teeth of mammals and those of the lower vertebrates is merely one 

 of parallel development, the adaptation being along similar lines, 

 and in no sense an indication of genetic affinity. 



The great development of the squamosal bone in Theriodonfc 

 reptiles is not alone, for somewhat similar proportions are found 

 in some other reptiles, — for example, in the Plesiosaurs, where the 

 squamosal is very large, and wrapped round the quadrate. In some 

 of the Dinosaurs, also (Torosaurus) , the squamosal has an enormous 

 development, while the quadrate remains of very moderate size. 



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 



