Sy6 



DIBRANCHIATE CEPHALOPODS. 



subcylindrical, conical, or fusiform " guard " or " rostrum." At its 

 anterior broad extremity the "guard" is hollowed out into a 

 conical excavation, termed the " alveolus " 

 (fig. 807, a). Within the alveolus, in well- 

 preserved specimens, is found the " phrag- 

 macone." This (fig. 807, p) consists of a 

 conical series of chambers, enclosed in a 

 thin proper wall (the " conotheca " of Hux- 

 ley), and separated from one another by 

 curved shelly partitions or " septa," which 

 are perforated by apertures for the passage 

 of the " siphuncle." The siphuncle is mar- 

 ginal, and traverses the middle of the ven- 

 tral wall of the phragmacone. The outer 

 or anterior chamber of the phragmacone is 

 of tolerably large size ; and the conotheca is 

 prolonged forwards on its dorsal side into a 

 horny or more or less calcified plate, known 

 as the " pro-ostracum." This corresponds 

 with the " pen " of the ordinary Cuttle- 

 fishes, and from its extreme tenuity is never 

 perfectly preserved, and, indeed, is com- 

 pletely wanting in the great majority of 

 specimens. 



The " guard " of the Belemnites consists 

 of prismatic calcareous fibres, which are 

 directed perpendicularly to the surface, and 

 radiate in all directions from an axial line, 

 which is not strictly central, but is some- 

 what nearer the ventral than the dorsal side. 

 The growth of the guard is effected by the 

 deposition of successive conical layers or 

 sheaths, which are secreted over the entire 

 surface, but are thickest behind, and become 

 gradually attenuated in front. The surface 

 of the guard is smooth ; or may be wholly 

 or partially granulated or wrinked ; or, again, 

 may be marked with branched vascular im- 

 pressions, which are especially conspicuous 

 on the ventral side. In many cases a well- 

 marked groove — the "ventral furrow" — 

 runs from the edge of the alveolus backwards on the ventral side, 

 extending for a short distance only, or reaching to the point of the 

 guard (fig. 806, c). In many cases the apical portion of the guard 

 shows two symmetrical grooves — the " dorso-lateral grooves " — which 





Fig. 807. — Diagram of Belem- 

 nite (after Professor Phillips). 

 r, Horny or shelly pen or 

 " pro - ostracum " ; fi, Cham- 

 bered "phragmacone" in its 

 cavity or "alveolus" («) ; g, 

 " Guard " 



