282 



MOLLUSCA. CEPHALOPODA 



or it may be granular, or furnished with ramifying vascular 

 impressions ; in some species there is a longitudinal groove on the 

 under surface. 



Fig. 121. Phragmocone and pro- 

 ostracum of Belemnites from 

 the Lias. Restoration by G. C. 

 Crick, a, phragmocone ; 6, 

 front border of phragmocone ; 

 c,last septum of phragmocone; 

 d, pro-ostracum. xf. 



Fig. 122. D'Orbigny's restoration 

 of a Belemnite (under surface), 

 showing the probable positions 

 of the guard, the phragmocone, 

 and the pro-ostracum. 



The phragmocone (figs. 120, b ; 121, a) is a hollow cone, part of 

 which fits into the alveolus at the broad end of the guard ; it is 

 divided into chambers by septa which are concave in front ; a 

 siphuncle (fig. 120, c) traverses the chambers at the under 

 margin ; at the pointed end of the phragmocone is a globular or 

 ovoid protoconch formed of calcareous material (figs. 120, 121). 

 The phragmocone is homologous with the entire shell of a Nauti- 

 loid or an Ammonoid ; in its conical form and simple sutures it 



