716 COMPARATIVE ANATOMY 



REPTILIA 



The special features in the turtle skull (Figs. 411, 413) are these: 

 teeth are absent, the maxillary, premaxillary, and dentary bones are cov- 

 ered with hard, chitinous sheaths which form the upper and lower 

 members of the cutting beak; the vomer is a single unpaired median 

 bone, and there are no lachrymals or ectopterygoids. The pterygoids 

 send wings of bone inward. The wings and the palatines form a con- 

 tinuous roof of the mouth ; the supraoccipital is prolonged backward 

 into a large, narrow process upon which are inserted the heavy neck 

 muscles. All of these bones, even the quadrate, are firmly united into 

 a solid cranium. There is only one occipital condyle. 



MAMMALIA 



The skull of the mammal (Figs. 413, 414) is more compact than that 

 of lower forms; consequently, it contains fewer elements than the skull 

 of reptiles. The following reptilian bones are not found in the adult 

 mammalian cranium : pre- and post- orbitals, pre- and post- frontals, 

 basi-pterygoids, quadrato-jugals, and supra temporals. The lower jaw 

 is reduced to a single pair of bones in the mammal, the angulare, splenial, 

 and articulare being absent. The latter is often said to have been drawn 

 in to form the malleus of the ear bonelets and the quadrate has been 

 drawn in to form the incus bonelet, while a remnant of the hyomandibu- 

 lar cartilage forms the stapes. The whalebone whale (baleen whale, 

 Fig. 392), shows the highest type of the so-called adaptive specialization 

 among mammals. Here the teeth are rudimentary and functionless 

 though present in the young. In the adult they are replaced by baleen. 

 The nostrils are paired, the skull symmetrical, the sternum is single, 

 while the ribs are one-headed and articulate only with the transverse 

 processes of the vertebrae. 



SUMMARY OF THE SKELETAL SYSTEM 

 THE DOGFISH 



The vertebrae develop at the intersection of the myosepta with the 

 mesenchyme that surrounds the notochord and neural tube. Each indi- 

 vidual vertebra is formed by the union of the two caudal halves of the 

 two sclerotomes of one segment with the cephalic halves of the two 

 sclerotomes of the next succeeding segment (Fig. 305). The vertebrae 

 therefore alternate. with the myotomes. 



As the vertebrae and ribs are first formed in cartilage produced by 

 the activity of mesenchyme, so also, bones which form later a,re true 

 cartilage bones. In the elasmobranchs, the entire skeleton is made up 

 of cartilage with only a slight impregnation of calcareous matter. 



Each vertebra begins as four pair of cartilages (called arcualia) 



