"] THE SPIXAL SKELETON. 63 



pophyses, is a character common to most animals of man's 

 class, but the centrum does not appear to have epiphyses 

 in the Sirenia, Monotremata, and in the animals below the 

 mammalian class. 



In those Mammals which have chevron-bones there are, 

 of course, ossifications which do not exist in man. 



FIG. 74.- CAUDAL VERTEBRA OF A CROCODILE. 



c, centrum ; s, neural spine ; t, transverse process ; z, pre-zygapophysis ; z\ post- 

 zygapophysis ; hy, hypapophysis, or ''chevron-bone."' 



In the lower Vertebrata the vertebral bodies may be at 

 first formed by superficial ring-like ossifications, as in many 

 Fishes ; and in some, as in the Carp, distinct lateral ossifi- 

 cations may exist, one on each side of the centrum. 



The walls of the two concave vertebral, articular cups may 

 ossify, making an hour-glass-shaped ossification, to whicn 

 concentric or radiating lamellae may be added, as in certain 

 Sharks. 



Cortical ossifications (i.e. of the fibrous sheath of the 

 notochord) may appear and coalesce with vertebrae, as in 

 the coccygeal vertebrae of the Frog. 



Sometimes, as in Lepidostren, bony neural arches may be 

 formed, and more or less embrace a permanently soft and 

 unossified chorda dorsalis. 



33. A bony case, like the THORAX of man, exists in all 

 Beasts and Birds, without exception. It also generally exists 

 in Reptiles, but is strangely modified in serpent-like Lizards, 

 and in Tortoises. In the former it is enormously drawn out, 

 and made imperfect below by the small development of a 

 sternum. In the latter it attains a maximum of solidity, and 

 enters into bony union with the dermal skeleton. 



