MOLLUSCA. CEPHALOPODA 



281 



SUB-OEDEK I. DECAPODA 



There are ten arms, eight of equal length and two 

 longer than the others; the latter can be more or less 

 completely retracted within pits. 

 The free ends of the arms are 

 swollen and suckers are usually 

 borne on those ends only. The 

 suckers are stalked and are provided 

 with a horny ring. An internal shell 

 is always present. 



Belemnites (figs. 120—122). The 

 shell consists of three parts — the guard 

 (fig. 120, a), the phragmocone (6), and the 

 pro-ostracum (fig. 121, d). 



The guard is solid and is much more 

 commonly preserved than the other parts ; 

 it varies considerably in shape and size, 

 being cylindrical, fusiform, conical, etc. 

 The end which was directed away from 

 the mouth is always pointed, and at the 

 other end there is a conical cavity or 

 alveolus. The guard varies in length 

 from one to fifteen inches. When sliced 

 transversely or longitudinally it is seen to 

 be formed of a number of layers (growth- 

 layers) arranged concentrically around an 

 axial line, which is not quite central but 

 is placed nearer the under surface ; it is 

 around this line that the first layers 

 were secreted ; the layers become some- 

 what thicker towards the pointed end 

 and thinner towards the broad end of the guard. Each layer is 

 formed of minute prisms of calcite, which are placed perpendicular 

 to the axial line, thus producing a radiating fibrous appearance in 

 cross-sections. The surface of the guard is sometimes smooth, 



Fig. 



120. Longitudinal 

 section of Belem- 

 nites, from the Ox- 

 ford Clay, a, guard; 

 b, phragmocone with 

 protoconch at the 

 apex ; c, siphuncle. 



X 2- 



