PHRAGMOPHORA. 



375 



Family i. Spirulid.e. — This family includes only the recent 

 genus Spirula (fig. 737, ^ and d, and fig. 805), in which the shell is 

 reduced to the phragmacone. This is coiled into a flat spiral, the 

 coils of which are not in contact, and it commences in an inflated 

 protoconch. The septa between the successive air-chambers are 

 concave ; and the siphuncle is marginal, and is placed on the ventral 

 or concave side of the shell. The siphuncle is completely enclosed 

 within a series of septal " necks," which are directed backwards, and 

 are long enough to reach from one septum to another. No fossil 

 representatives of the genus have hitherto been recognised. 



Family 2. Belemnitid^e. — In this family the shell has the form 

 of a conical " phragmacone," with a ventral siphuncle (fig. 806, a), 

 which is lodged within a solid fibrous calcareous sheath or " guard " 

 (6), and which has its dorsal margin prolonged into a thin horny or 

 shelly plate or " pro-ostracum " (c), corresponding with the " pen " 





Fig. 806.— A, Restoration of the animal of the Belemnite ; b, Diagram showing the complete 

 skeleton of a Belemnite, consisting of the chambered phragmacone (a), the guard (/>), and_ the horny 

 pen (c); c, Specimen of Belemnites canaliculaties, from the Inferior Oolite. (After Phillips.) 



of the Chondrophora. The arms in the Belemnitida were furnished 

 with booklets, and an ink-bag was present. The members of this 

 family are all extinct, and range from the Trias to the Miocene 

 Tertiary. 



The most important genus in this family is Belemnites itself, in 

 which the portion, of the shell most commonly preserved is the 



