404 



Prof. Huxley on the 



[Mar. 6, 



Unless the like should prove to be the case in the Dicynodonts, it is 

 in the Mammalia alone that the subsacral portion of the ilium elongates 

 backwards, carrying with it the pubis and the ischium, between which 

 a large rounded obturator fontanelle is developed. 



These facts appear to me to point to the conclusion that the Mam- 

 malia have been connected with the Amphibia by some unknown " pro- 

 mammalian " group, and not by any of the known forms of Sauropsida ; 

 and there is other evidence which tends in the same direction. 



Thus, the Amphibia are the only air-breathing Vertebrata which, like 

 Mammals, have a dicondylian skull. It is only in them that the 

 articular element of the mandibular arch remains cartilaginous ; 

 while the quadrate ossification is small, and the squamosal extends 

 down over it to the osseous elements of the mandible ; thus affording 

 an easy transition to the mammalian condition of these parts. 



The pectoral arch of the Monotremes is as much amphibian as it is 

 sauropsidan ; the carpus and the tarsus of all Sauropsida, except the 

 Chelonia, are modified away from the Urodele type, while those of 

 the Mammal are directly reducible to it ; and it is perhaps worth notice, 

 that the calcar of the frogs is, in some respects, comparable with the 

 spur of the Monotremes. 



Finally, the fact that, 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, 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 Sauropsida 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. 



The discovery of the intermediate links between Reptilia and Aves, 

 among extinct forms of life, gives every, ground for hoping that, 

 before long, the transition between the lowest Mammalia at present 

 known and the simpler Vertebrata may be similarly traced. The 

 preceding remarks are intended to direct attention to the indications 

 of the characters of these promammalian Vertebrata, which the 

 evidence at present forthcoming seems to me to suggest. 



In the relatively large size of the brain, and in the absence of 

 teeth, the only existing representatives of the Ornithodelphia present 

 characters which suggest that they are much modified members of 

 the group. On comparing the brain of Echidna, for example, with 

 that of many Marsupialia and Insectivora, its relative magnitude 

 is remarkable : and, in view of the evidence which is now accumu- 

 lating, that the brain increases in size in the later members of the 

 same series of Mammalia, one may surmise that Echidna is the last 



