92 



DIBRANCHIATE CEPHALOPODS. 



With these exceptions, however, no Tertiary examples of tlie 

 family are known, and the group has no living representa- 

 tives. The following are the more 

 important genera belonging to this 

 family : — 



a. Bdcmnitcs. — The skeleton of the 

 Belemnite consists of a sub-cylindrical, 

 longer or shorter, fibrous body (figs. 

 477, 478), which is termed the 

 " rostrum " or " guard." The length 

 of the guard varies very much in 

 different cases, and it is the part of 

 the Belemnite which is most com- 

 monly found in a fossil condition. 

 At the front or broad end, the guard 

 is hollowed out into a conical ex- 

 cavation, which is termed the " al- 

 veolus." Within the alveolus, in 

 perfect specimens, is contained the 

 " phragmacone." This consists of a 

 conical series of chambers, separated 

 from one another by curved shelly 

 partitions or septa, which are per- 

 forated by apertures for the passage 

 of the " siphuucle." The siphuncle 

 traverses the middle of the ventral 

 wall of the phragmacone, and the 

 whole series of chambers is enclosed 

 in a thin shell- wall (the " conotheca " 

 of Huxley). Anteriorly the conotheca 

 or investment of the phragmacone is 

 prolonged forwards into a horny or 

 shelly plate, which corresponds with 

 part of the " pen " of the Calamaries, 

 and which is termed the "pro-ostra- 

 cum" (fig. 477, r). The form of the " pro-ostracum " varies 

 greatly in different cases, and it affords important char- 

 acters in the discrimination of specific and generic forms in 

 the Belemnitidcc. Owing, however, to its extreme tenuity, 



II 





Fig 477. — Diagi'ain of Belem- 

 nite (after Professor Phillips). 

 r, Homy or shelly pen or 

 " pro - ostracum ; " f. Cham- 

 bered "phragmacone" in its 

 cavity (a) or " alveolus ; " g, 

 " Guard." 



