Characteristics of the Vertebrata. xli 



tlie three most anteriorly placed of these that cartilaginous supports 

 have been observed to be developed. 



SuB-DivisTON, Sauropsida. 



The two Classes Aves and Reptilia are united into a single 

 province, that of the ' Sauropsida,' by the possession of the following 

 characteristics which distinguish them both from the other sub- 

 division of Allantoidea, the class Mammalia. Their integumentary 

 system always developes either feathers or scales ; their skull is 

 articulated by a single occipital condyle to the cervical vertebrae ; 

 they have their ankle-joint interposed between the proximal and 

 distal bones of the tarsus, and not between the distal extremity of 

 the lower leg and the proximal tarsal bones; they have also a 

 single auditory ossicle, the stapes ; the malleus of Mammals being 

 represented by their os quadratum, and the incus by their supra- 

 stapedial process. Their blood corpuscles are oval and nucleated. 

 They never have the single aorta turning over the left bronchus, 

 nor a perfect diaphragm, nor a corpus callosum, nor mammae, all 

 of which structural arrangements are found in all Mammals. 



Division, Anallantoidea- 



The Anallantoidea comprise the two Classes, Amphibia and Pisces, 

 and the province thus constituted is known as that of the ' Ichthy- 

 opsida,'' as well as by the names of ' Branchiata^ and ' Anamniota,' 

 given above. They differ from the Abranchiate province in the 

 following particulars in addition to those which their various names 

 imply. Their integument never develoj)es any epidermic skeleton, 

 except in a few Amphibia (see p. ;^^, infra), and in some Fishes the 

 bony structures of the cutis may cause the total or almost total 

 disappearance of the cuticle. The axis of the head and vertebral 

 column form one continuous line ; the basicranial bones are, except 

 in some fishes which have not an osseous skeleton, underlaid by a 

 large parasphenoid. The basi-occipital may be rudimentary or 



