382 ZOOLOGY. 



paralleled by the Amphibians 5 and urges lliat onlj' in the Amphibians 

 is the articular element of the mandibular arch persistently cartilaginous, 

 while the quadrate ossification is small, and "the squamosal extends 

 down over it to the osseous elements of the mandible, thus aflbrdiug 

 an easy transition to the mammalian condition of these parts." Further, 

 "the pectoral arch in the Monotremes, it is claimed, is as much like 

 that of the Ami^hibians as it is that of any Sauropsidan; and the carpus 

 and tarsus of the mammals are directly deducible from the urodele 

 amphibian type. (It is also suggested that the calcar of the frogs is in 

 some respects comparable with the spur of the Monotremes, though this 

 is rather irrelevant.) Finally, it is said "in all Sauropsida it is a right 

 aortic arch which is the main conduit of arterial blood leaving the heart, 

 while in mammals it is a left aortic arch which performs this office." 

 This discrepancy is thought to be "a great stumbling-block in the way 

 of the derivation of the Mammalia from any of the Sauropsida j but if 

 we suppose the earliest forms of both the Mammalia and the Sauropsida 

 to have had a common amiihibiau 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 i>redominant feeder of the arterial system." 



In later publications* Professor Huxley has reiterated his opinions. 

 The ancestors of mammals or " Promammals " would be distinguished from 

 the Sauropsida by the possession of two occipital condyles and the en- 

 larged size and functional development of the left aortic arch, while 

 they would "probably be no less difterentiated from the Amphibia by 

 the i^resence of an amnion and the absence of branchiae at any x^eriod of 

 life." He does not doubt that " when we have a fuller knowledge of the 

 terrestrial vertebrata of the later Palieozoic epochs, forms belonging to 

 this stage will be found among them." A type like the one thus sup- 

 posed without the amnion and corpus callosum, and provided with func- 

 tional branchiae, would find a place among the Amphibians. In the line 

 of descent, from the primitive vertebrate to the mammals, neither the 

 reptiles nor birds would intervene, they, according to Huxley, represent- 

 ing "as it were side-tracks starting from certain points of that line" in- 

 termediate between the amphibian and mammalian stages of evolution. 



MAMMALS OF THE AMEEICAN JUEASSIC. 



Previous to 1827, we had very little definite information respecting 

 the former existence of mammals in the Jurassic epoch of North Amer- 

 ica, although Emmons had indicated that a form of that class had left 

 remains in certain beds in North Carolina. In the last several years, 

 however, quite a number of species have been indicated, and based upon 

 lower jaws or fragments thereof, found in various beds in the "Western 

 Territories. Up to the present time 13 nominal species, representing 



* Prof. Huxley on Evolution. JSfature, vol. xxiii (1), pp. 203-204, Dec. 30, 1880; (2), pp. 

 827-231, Jan. 6, 1681. 



