126 ANATOMY OF VERTEBRATES. 



suprascapula and scapula together represent the rib or pleur- 

 apophysis 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 scapula ; the middle part developes backward a broad plate 

 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 Silurida? the 

 coracoids expand below, and are united together by a dentated 

 suture. In all Fishes they support 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 dividing 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 {Chelone, fig. 89) be compared with 

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

 lowest bone, l, 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 pro- 

 tect the sides of the epencephalon. These, 



Back view of cranium, Turtle . . •■ , , 



moreover, transmit the hypoglossal and 

 vagal nerves, develope each an articulation for the neurapophyses 

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

 arch, 3. AYe have, thus, unmistakeable 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 produced on its outer surface for 

 the insertion of the ' biventer cervicis ' and ' complexus ' ; it is the 

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

 bears the same general relation to the hindmost vertebral segment 

 of the skull which the mastoid, s, does to the next segment in 

 advance. 



