SKELETON 



173 



second dorsal fin is extended, and its simple rod-like radials appear 

 to be in a primitive undifferentiated condition. The dorsal (axial) 

 lobe of the heterocercal caudal fin is much elongated and tapers 



A. 



B 



FIG. 137. 



,s'///(ir7,,, 17/n polyspondyla, Ag. Enlarged views of, A, vertebral rings ; B, denticles, b.p, 

 basal plate ; c, projecting spine ; -no, cavity for notochord ; v, vertebral ring. 



into a long whip-like end. In Harriotta the long tail is straight, 

 and scarcely betrays its original heterocercal character (Fig. 145). 

 The anal fin shifts far back to near the caudal ; it is often very 

 small, and is not present in Harriotta. 



FIG. 13S. 



Ventral view of the pelvic girdle and tins of Chinuum monstrosn, L., 9 The skeleton is com- 

 pletely exposed on the left side. l>, basipterygium ; </>, web of light tin with ceratotrichia ; p, 

 pelvic cartilage ; pi-.i; preaxial radials ; jit.r, postaxial radials. (From Quart. Joiirn. Mies. .Sri.) 



The pectoral girdle is formed of stout cartilages fused in the 

 middle ventral line. They support the skeleton of the fin, which 

 is essentially of the typical Selachian kind, with many fan-like rays 

 attached to a nietapterygium (Fig. 134). The pelvic fin has a very 

 similar but smaller skeleton (Fig. 138). The pelvic girdle, on the 



