126 ANATOMY OF VERTEBRATES. 



sujDrascapula and scapula together represent tlie rib or pleur- 

 apojihysis of the occipital vertebra ; they are always confluent in 

 the Siluroids. The lower bone ' coracoid,' ib. 52, completes the 

 arch. In the Cod its pointed upper extremity projects behind 

 the scapvila ; the middle 2)art developes backward a broad jjlate 

 giving attachment to the radiated appendage of the arch : the 

 lower end bends inward and forward gradually decreasing to a 

 point, which is usually connected to that of the opposite coracoid 

 by ligament, and also to the urohyal. In the Siluridte the 

 coracoids expand below, and are united together by a dentated 

 suture. In all Fishes they sujjport and defend the heart, and form 

 the frame or ' sill ' against which the opercular and branchiostegal 

 doors shut in closing the branchial cavity : they also give attach- 

 ment to the aponeurotic diaphragm di'\'iding the pericardial from 

 the abdominal cavity. 



The bones of the head being in completest number, de- 

 parting least from the vertebral pattern, and susceptible of the 

 most intelligible definitions in the class of Fishes, afford the best 

 basis for determining their homologies and fixing their nomencla- 

 ture in the higher vertebrate series. 



§ 31. Skull of Chelonia. If the back part of the skull of a 

 Turtle (Chclonc, fig. 89) be compared with 

 that of a Cod, fig. 77, it will be seen that the 

 lowest bone, i, offers an articular surface 

 for the centrum of the atlas, passes for- 

 ward, expanding, to articulate with the 

 basisphenoid, supports the 'medulla ob- 

 longata,' and is suturally articulated above 

 to the pair of bones, fig. 89, 2, 2, which j^ro- 

 tect the sides of the eponcephalon. These, 

 moreover, transmit the hypoglossal and 

 vagal nerves, develope each an articulation for the neurapophvses 

 of the atlas, and converge above to support the keystone of the 

 arch, 3. We have, thus, unmistakcable characters of the basi- 

 ex- and super-occipitals ; there is also a bone, 4, wedged between 

 the ex- 2, and super- 3, occipitals mesially, and joined laterally 

 to the mastoid, 8 : excavated on its inner surface by the postero- 

 external semicircular canal, and iiroduccd on its outer surface for 

 the insertion of the 'bi venter corvicis' and ' com[)lcxus' ; it is the 

 homologue of the paroccipital (' occipitale cxtcrne,' Cuvier), and 

 bears the same general relation to the liindmost vertebral segment 

 of the skull wliich the mastoid, s, docs to the next segnieiit in 

 advance. 



Click view of crauiiim. Turtle 



