DIBRANCHIATE CEPHALOPODS. 



297 



Trias to the Chalk. The following are the more important 

 genera belonging to this family : 



a. Belemnites. The skeleton of the Belemnite consists of a 

 sub - cylindrical, longer or shorter, 

 fibrous body (fig. 266), 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 

 commonly found in a fossil condi- 

 tion. At the front or broad end, the 

 guard is hollowed out into a conical 

 excavation, which is termed the 

 " alveolus." Within the alveolus, 

 in perfect specimens, is contained 

 the " phragmacone." This consists 

 of a conical series of chambers, se- 

 parated from one another by curved 

 shelly partitions or septa, which 

 are perforated by apertures for the 

 passage of the " siphuncle." 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). Anteri- 

 orly the conotheca or investment of 

 the phragmacone is prolonged for- 

 wards into a horny or shelly plate, 

 which corresponds with part of the 

 " pen" of the Calamaries, and which 

 is termed the " pro-ostracum " (fig. 

 266, r). The form of the "pro- 

 ostracum " varies greatly in different 

 cases, and it affords important char- 

 acters in the discrimination of spe- 

 cific and generic forms in the Belem- 

 nitid(Z. Owing, however, to its ex- 

 treme tenuity, it is very rarely found 

 preserved in a fossil condition, and 

 its value to the working palaeonto- 

 logist is thus greatly reduced. 



Not only is the internal skeleton 

 of the Belemnite known, but various specimens have been dis- 

 covered, from which much has been learnt as to other points 



Fig. 266. Diagram of Belem- 

 nite (after Professor Phillips), r 



pro- 



Horny or shelly pen or 

 ostracum; "^Chambered "phrag- 

 macone" in its cavity (a) or "al- 

 veolus ; " g " Guard. 



