MOLLUSCA — CEPHALOPODA. 



155 



animal is the familiar cuttle-bone (Fig. 85 e). Viewing it ^^^^ 

 from the back one sees at its end a small point (the mucro) ipable-cas 

 corresponding to the guard, and in front stretches a broad i. 

 shelly plate,' like a pro-ostracum. This, however, when 

 viewed from the other side, is seen to be covered by a mass 



c 



ah d e f g h 



Fig. 85. — The Belemnite and its Descendants. Sections through the 

 middle of the shells, all in the same position, a, a belemnite ; b, Spiru- 

 lirostra; c, Spirula ; d, Belosepia ; e, Sepia; f, Belemnoteuthis ; 

 g, Conoteuthis ; h, Ommastrephes ; i, a squid. In all figures, p.o. is 

 pro-ostracum ; si, siphuncle ; and g, guard. The shell-wall and septa 

 are represented by thick black lines. 



of thin shelly plates, which correspond to the septa more 

 clearly seen in Belosepia. 



Another Eocene fossil that appears to be descended from 

 the belemnites is Spirulirostra (Fig. 85 6). In some of the 

 later belemnites may already be observed a shortening of the 

 guard and a curvature of the phragmocone, processes which 

 tend to reduce the unncessary length of the shell. In 



