ORIGIN 83 



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 Saur- 

 opsida 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." 

 Subsequently Professor E. D. Cope l in a suggestive paper called 

 attention to the remarkable resemblances to the Monotremes pre- 

 sented by the skeleton of that group of early secondary reptiles 

 which he then designated the Theromorpha, but which may be 

 included in the Anomodontia of Sir R. Owen, and came to the 

 conclusion that in that group we have the true ancestors of the 

 Mammalia. This conclusion was, however, disputed by Dr. Baur, 2 

 who considered that the Anomodontia were too specialised to have 

 been the actual progenitors of the Mammalia, and that they should 

 rather be regarded as a divergent branch of the stem which had given 

 origin to the Mammalia. Since that date observations made on 

 the structure of the South African Anomodonts have shown such 

 an intimate connection between that group and the Labyrinthodont 

 Amphibians, that there can be no hesitation in regarding the one 

 as the direct descendant of the other ; and we may probably regard 

 the Mammalia as having originated from the same ancestral stock 

 at the time the Amphibian type was passing into the Reptilian. 

 From this point of view, some of the mammalian features found in 

 the more specialised Anomodonts may probably be regarded as 

 having been acquired during a parallel line of development. 



Both the Anomodontia and the Mammalia differ from the 

 Amphibians in the loss of the splint- like parasphenoid which 

 underlies the basisphenoid axis of the skull, and by the ossification 

 of that axis ; but while the former have become monocondylic by 

 the participation of the basioccipital in the support of the cranium, 

 the latter retain the Amphibian dicondylic plan. The skull of the 

 Anomodonts presents mammalian resemblances not found in any 

 other Reptiles, this being especially noticeable in the region of the 

 squamosal ; and it is only in this group and mammals that the 

 temporal or zygomatic arch is a squamoso-maxillary one (see p. 

 37). The resemblance between the pectoral and pelvic girdles 

 of the Anomodonts and those of the Monotreme Mammals is 

 noticed under the head of the latter, where reference is also made 

 to the similarity in the structure of the humerus in the two groups. 



1 " The Relations between the Theromorphous Reptiles and the Monotreme 

 Mammalia," Proceedings of the American Association for the Advancement of 

 Science, vol. xxxiii. p. 471 (1885). 



2 "On the Phylogenetic Arrangement of the Sauropsida," Journal of 

 Morphology, vol. i. pp. 93-104 (1887). 



