272 AN INTRODUCTION TO THE STUDY OF FOSSILS 



solid, and when examined in section is seen to be composed 

 of prisms of calcite radiating from an axial line slightly nearer 

 the ventral surface. The phragmocone fits into the hollowed 

 end of the guard — the alveolus, and is divided into chambers 

 by septa which are perforated by the ventrally placed si- 

 phuncle; this is followed by the calcified, blade-like proostra- 

 cum. The proostracum corresponds to the pen of the squid 

 and probably in life afforded dorsal protection to the viscera. 

 The phragmocone corresponds to the entire shell of the Tetra- 

 branchiata. 



As an index fossil of the Jurassic and Cretaceous it is scarcely 

 less important than the ammonites. The name, from the 

 Greek belemnos, a dart, was given by Agricola in 1546. It is 

 commonly called '' arrowhead " or " finger stone " from the 

 shape, and " thunderbolt " or '' thunder stone " from its 

 supposed origin. 



1. Split the broad end of a specimen longitudinally from 

 the ventral groove backwards (if the narrower end sphts likewise, 

 this should be glued into place again). Sketch view showing 

 this section. Label alveolus, guard. 



2. Note the calcareous, prismatic fibers of which the guard is 

 composed, and the successive conical layers by means of which 

 the guard increases in size. Make a cross section showing this 

 increase. 



3. The walls of the alveolus usually show faint impressions 

 of the chambered phragmocone, the siphuncle of which was 

 located ventrally, i.e. upon the side next the ventral groove. 

 Indicate upon the first sketch the ventral and dorsal sides. 



4. What is the significance of the name Belemnites ? 



In the genus Sepia (Tertiary to present) the shell is likewise 

 internal but extends the entire length and breadth of the body. 

 It is without differentiated phragmocone and guard, but consists 

 essentially of proostracum or " pen." The projecting point 

 may represent the guard. This is the familiar cuttle bone of 

 commerce. Sepia officinalis is very common in the Mediter- 

 ranean. The coloring matter from its ink sac is insoluble in 



