180 MOLLUSC A. CEPHALOPODA. 



layers are not of uniform thickness along the whole length 

 of the guard, they become thicker towards the posterior 

 end and thinner towards the anterior. Each layer is 

 formed of minute prisms of calcite, which are placed 

 perpendicular to the axial line, thus producing a radiating 

 fibrous appearance. The surface of the guard is some- 

 times smooth, or it may be granular, or furnished with 

 ramifying vascular impressions ; in some species there is 

 a longitudinal groove on the ventral surface. 



The phragmocone (6) is a conical tube which fits into 

 the alveolus at the anterior end of the guard, it is divided 

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

 siphuncle (c) traverses the chambers along the ventral 

 margin. The wall of the phragmocone (sometimes termed 

 the conotheca) is very thin, and in well preserved specimens 

 the dorsal surface (or sometimes the lateral) is found to 

 be produced in front into a large laminar expansion 

 (d)] this prolongation is known as the pro-ostracum, 

 and corresponds to the "pen" of the cuttle-fish. The 

 suckers on the tentacles were provided with horny hooks, 

 which are occasionally preserved fossil ; the ink-sac and 

 mandibles have also been found in a few specimens. Lias 

 to Upper Cretaceous. 



Belemnitella. This differs from the preceding genus 

 in having a slit in the guard on the ventral side of the 

 alveolus. There are well-marked vascular impressions on 

 the ventral surface. Chalk. 



SUB-ORDER. OCTOPODA. 



There are eight arms only ; the suckers are sessile and 

 possess no horny ring. The shell is rudimentary or 

 absent. This sub-order is very poorly represented in the 



