THE DOGFISH 31 



two ventral. The dorsal elements are the pharyngobranchial and 

 epibranchial cartilages. The former run obliquely backward, ter- 

 minating dorsally beneath the spinal column, the two posterior 

 pharyngobranchials being fused together; the latter are vertical 

 in position and correspond to the hyomandibular cartilage. The 

 ventral elements are the ceratobranchial and hypobranchial carti- 

 lages, of which the former articulate with the epibranchials in the 

 midlateral plane and the latter run obliquely backward to meet 

 their fellows of the opposite side in the midventral line. The 

 hypobranchials are not all alike. In the smooth dogfish the two 

 posterior pairs and in the spiny dogfish the three posterior pairs 

 are fused together and with the fifth ceratobranchials, forming a 

 broad median plate called the basibranchial cartilage. 



The epibranchials and ceratobranchials of the first, second, 

 third, and fourth branchial arches bear gill rays. 



Along the outer sides of the second, third, and fourth bran- 

 chial arches are three pairs of supernumerary cartilages called 

 the extrabranchials. 



Exercise 37. Draw a semidiagrammatic side view of the visceral 

 skeleton. 



Exercise 38. Draw a similar ventral view. 



The Appendicular Skeleton; the Median Fins. The median fins 

 are an anterior dorsal and a posterior dorsal fin, an anal fin (in the 

 smooth but not in the spiny dogfish), and a caudal fin. The skele- 

 ton of the dorsal fins consists of a row of cartilaginous fin rays, 

 the bases of which are often fused together, while at the outer 

 ends are a number of small plates of cartilage. On each side of 

 these fin rays, and extending beyond them, is a series of slender, 

 horny fibers which develop in the skin and belong to the exo- 

 skeleton. The other median fins possess these fibers alone, the 

 cartilaginous elements being much reduced or entirely absent. 



The Paired Fins. These consist of the pectoral fins and the 

 ventral fins, which are supported by the pectoral and pelvic girdles 

 respectively. 



The pelvic girdle is an elongated plate of cartilage which extends 

 transversely across the body in front of the anus and articu- 



