44 COMPARATIVE ANATOMY 



Reference has already been made to the dermal bones amongst 

 fossil Fishes, Amphibians, and Reptiles, and certain of these in 

 the antero-ventral region of the trunk are of special interest, as 

 they are represented in certain Reptiles by a bone known as the 

 episternum, which underlies the sternum. Though always arising 

 as a paired structure, the episternum, which is present, e.g., in 

 Palaeohatteria, most Lizards (Fig. 56) and Crocodiles, forms in the 

 adult an unpaired plate of varied form. It is wanting in 

 Chamaeleo, Anguis, Ophidia, and Chelonia. 1 



Birds. Reduced abdominal ribs occur in the primitive 

 Archa^opteryx, otherwise no fossil or existing Birds possess a 

 dermal exoskeleton, and no independent elements corresponding 

 to an episternum can be recognised even in the embryo: they 

 have evidently long ago disappeared. 



Mammals." Armadillos are the only Mammals possessing a 

 bony exoskeleton, 3 which consists of a series of five movable 

 transverse bony scutes covering the head, neck, and body, and of 

 smaller plates on the tail and limbs. Sparse hairs occur between 

 these plates. It is very doubtful whether this exoskeleton has 

 been derived from that of Reptiles: more probably it, like the 

 horny exoskeleton of Manis (p. 31), has arisen secondarily, and in 

 consequence of its development the hairs have become reduced. 

 In Glyptodon, a large fossil member of this group, the dermal 

 plates were firmly united together to form a shield which covered 

 the whole body. 



2. ENDOSKELETON. 



Under the term exoskeleton are included the bony parts 

 which, as a rule, remain throughout life in connection with the 

 integument : the endosMcton consists mainly of cartilaginous and 

 bony parts, all of which have a deeper position. The cartilaginous 

 portions, which in their entirety constitute the primordial endo- 

 sMcton, have undoubtedly from the first arisen in this position, 

 and for a long period formed, together with the notochord, 

 the entire internal skeleton, as they practically do at the present 

 day in Elasmobranchs as well as in Cyclostomes. As already 



1 Unless the element of the plastron marked E in Fig. 33 is to be interpreted 

 as such. 



2 A certain part of the anterior end of the sternum, as well as certain carti- 

 laginous and bony elements in the region of the sterno-clavicular articulation, are 

 sometimes said to correspond to the last remains of the dermal episternum of 

 lower forms ; but as further proofs are required before such a homology can be 

 definitely accepted, the term proxttrnum has been proposed to include the 

 elements in question (cf. under .Sternum). 



3 It is possible that the peculiar horny tubercle in the region of the dorsal 

 fin in certain Cetaceans may represent the last vestiges of a dermal bony arma- 

 ture such as was present in the extinct Zeuglodon. 



