322 OUR COMMON BRITISH FOSSILS. 



part is usually hollow ; sometimes, however, it is 

 occupied by an inverted conical object, divided into 

 nearly horizontal chambers. The latter is called the 

 phragmacone. Its chambers are usually traversed by 

 a tube or siphuncle, as in those of the Nautilus, and 

 the internal parts of the extinct animals which formed 

 the Belemnites were chiefly contained within the 

 last-formed chamber, as the body of the whole animal 

 is in the Nautilus. The true Belemnites only occur in 

 Secondary rocks of this country. In the Chalk we 

 have a sub-genus termed Belemnitella, which may be 

 distinguished from the former by the slit in the upper 

 part of the guard. It is clearly seen in that most 

 abundant and widely distributed Chalk species 

 Belemnitella mucronata. 



In the Lias and Oolite, Belemnites are marvel- 

 lously abundant. Near Whitby the limestone fre- 

 quently appears as if wholly composed of them. 

 Scores are often seen on the surface of a single slab. 

 No fewer than one hundred and fifteen species have 

 been described from the Oolitic rocks of Great Britain 

 alone. There is as great variety in their sizes as in 

 the other families of Cephalopods. For instance, we 

 have some species in the Lias whose " guards " are a 

 foot and a half long ; and others, like Belemnites 

 minima, found plentifully in the Lower Cretaceous 

 strata and particularly in the Gault only about 

 an inch long. 



Genuine cuttle-fishes, allied to existing genera, 



