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! 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,? 
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. 98-104 (1887). 
