\6 



CLASS PISCES. 



column and the cranium ; the posterior aspect of the basioccipi- 

 tal in other groups forming a cup like that of a vertebral centrum. 

 The mode in which the vertebral column terminates posteriorly is 

 of considerable importance in classification. The most primitive 



Fig. 840. — a, Polyfiterus, and b, Osteolepis (Ganoids), to show diphycercal caudal fin. 

 a, Pectoral; b, Ventral; c, anal ; d, Dorsal fin. Reduced. 



type occurs in the Cyclostomi, Dipnoi, and many Ganoids, where 

 the notochord continues to the extremity of the body, and is sym- 

 metrically surrounded by the caudal fin, as in fig. 840 ; this type is 

 known as diphycercal. In the other, or heterocercal type, the notochord 



-a, Sword-fish ; b, Sturgeon, to show masked (homocercal) and typical 

 heterocercal tail. 



is bent upwards, owing to the greater development of the lower as 

 compared to the upper half of the tail. This feature may be observ- 

 able externally, as in the Sturgeons (fig. 841, b) and Sharks ; or may 

 be masked, as in the majority of Teleostean Fishes (fig. 841, a), by 

 the symmetrical arrangement of the fin-rays. The skeleton (fig. 



