MOLLUSC A. CEPHALOPODA. 



179 



SUB-OKDER. DECAPODA. 



There are ten arms, eight of equal length and two 

 longer than the others : the suckers are stalked and are 

 provided with a horny ring. An 

 internal shell is always present. 



The genera Spirula and Sepia 

 already mentioned belong to this 

 sub-order; the former does not 

 occur fossil and the latter only 

 rarely. Allied to Sepia is the genus 

 Belosepia found in the Eocene. The 

 most important fossil form is Belem- 

 nites. 



Belemnites. The shell in this 

 genus consists of two parts the 

 guard (fig. 51 a) and the phragmo- 

 cone (6). The former is solid and 

 is much more commonly preserved 

 than the latter; it varies consider- 

 ably in shape and size, being cylin- 

 drical, fusiform, conical, etc. The 

 posterior end is always pointed; at 

 the anterior end there is a conical 

 cavity, termed the alveolus. The 

 length of the guard varies from 

 one to fifteen inches. 



When sliced transversely or longi- FIG. 51. Diagram- 

 tudinally it is seen to be formed of a section of Belemnites. 



. fr' . i i a, guard; b, phragmo- 



number of layers arranged concen- cone . c< siphuncle tra . 

 trically around an axial line, which versing the chambers of 

 is not quite central but is placed the phragmocone; d, pro- 

 nearer the ventral surface; these ostracum - 



122 



