PHYLUM MOLLUSCA 



301 



pletely covered by the mantle. Its shell has been found 

 cast ashore on Nantucket. 



In the other Dibranchiata the shell 

 may consist of three parts, — a horny 

 pen or pro-ostracum, a calcareous 

 guard, and a part termed the phrag- 

 mocone. The last, which alone repre- 

 sents the shell of Spirula, has the 

 form of a cone divided internally by 

 a series of septa perforated by a si- 

 phuncle. These parts are most com- 

 pletely developed in the extinct genus 

 Belemnites, in which the shell con- 

 sists of a straight, conical, chambered 

 phragmocone, with a siphuncle, en- 

 closed in a calcareous 

 sheath, the guard, pro- 

 duced into a horny or 

 calcareous plate, the 

 pro-ostracum. In the 

 cuttle-fish of the Medi- 

 terranean Sea (Sepia) the shell is a leaf-like 

 body, with a rounded and comparatively 

 broad oral end, and a narrower aboral pro- 

 vided with a sharp projecting spine. The 

 main mass of the shell consists of numerous, 

 closely arranged, thin laminae of calcareous 

 composition, between which are interspaces 

 containing gas. The spine-like projecting 

 point represents the guard, and the main 

 substance of the shell is to be looked upon 

 as the pro-ostracum and phragmocone, the 

 septa of the latter being represented by the calcareous 



Fig. 185. — Spirula peronii, 

 lateral view. d t terminal 

 sucker; f t funnel; s,, s 2 , 

 projecting portions 01 the- 

 shell, the internal part of 

 which is indicated by dotted 

 lines. (From Cooke.) 



Fig 186. — Shell of 

 Sepia cultrata, 

 posterior view, 

 reduced. 



