284 MOLLUSCA. CEPHALOPODA 



of tbe phragmocone, whilst the guard is reduced to a small pointed 

 process (or mucro) at the end. Spirula has not been found fossil. 



Belemnitella. Similar to Belemnites. Guard cylindrical, 

 with a slit at the under side of the alveolus. Distinct vascular 

 impressions on the under surface of the guard. Upper Chalk. Ex. 

 B. mucronata. 



Actinocamax ( = Atractilites). Similar to Belemnites. Front 

 part of guard either conical or broadly funnel-shaped, and either in 

 contact with the protoconch only or surrounding only the apical 

 part of the phragmocone. Front part of guard often fragile and 

 foliaceous owing to imperfect calcification. Chalk. Ex. A. plenits, 

 Lower Chalk ; A. quadrattis, Upper Chalk. 



Aulacoceras. Similar to Belemnites. Guard relatively 

 short, with a groove extending down each side. Phragmocone 

 much longer than the guard, with siphuncle at the margin, and 

 septa rather widely separated ; surface of phragmocone with 

 longitudinal lines. Pro-ostracum unknown. Upper Trias. Ex. A. 

 reticulatum. 



Belemnoteuthis. Guard much reduced, forming a thin 

 layer over the phragmocone, with a groove on the upper side 

 starting from the pointed end. Phragmocone broad, with numerous 

 septa, a marginal siphuncle, and septal necks. Pro-ostracum rela- 

 tively small, seldom preserved. Ten arms bearing hooks. The ink- 

 sac is sometimes found. Chiefly Oxfordian. Ex. B. antiqua. 



SUB-ORDER II. OCTOPODA 



There are eight arms only; the suckers are sessile and 

 possess no horny ring. The shell is rudimentary or absent. 

 Octopus and Argonauta are well-known examples of this 

 group. The Octopoda, as might be expected from the 

 general absence of a shell, are very poorly represented in 

 the fossil state ; the earliest known form is Palceoctopus 

 from the Chalk of Lebanon. Argonauta has been found 

 in the Pliocene Beds. 



