INTERPRETATION OK THE SHELL IX RECENT CEPHALOPODS. 289 



preserved, and as is also shown by the so-called 

 externa] surface of bhe rostrum of many 

 Belemnites.* 



Attempts have been made to deduce the 

 constituent parts of the shell oi Sepia from 

 bhe above parts of the Belemnite shell 

 (Volt/. Riefstahl, No. 39). The shell of 

 Sepia, or, as it isgenerallj bermed, the cuttle- 

 bone, is very complicated. The whole forms 

 an oval shield-like structure, which, Eor the 

 most part, is biconvex, but in the more dorsal 

 region it becomes concave on its posterior 

 or inner side, its shape is well shown in 

 Pigs. 137, 138 A. Its antero-dorsal surface 

 is covered externally by a roughly calcified 

 shagreen-like layer, the outer layer, under 

 which is a deposit of horn-like matter (con- 

 ehyolin), the middle layer, the latter being 

 freely exposed at the margin of the shell 

 i Fig. 137, mp). Dorsally, the shell is 

 produced into a small pointed structure 

 (Figs. 137 and 138, k. ,/> which consists 

 essentially of a prolongation of the outer 

 calcified layer but has become covered by a 

 secondary development of horny (oonchyolin) ; 

 matter which is quite distinct from the 

 horny middle layer. This calcareous spine 

 may in some species project freely on to 

 the exterior (S. andreanoides). On the 

 posterior or internal surface of the shell is 

 a prominent swelling produced by a great 

 deposit of calcareous material arranged in 

 thin oblique layers (Pig. 138 A, w), separated 

 from one another by air-spaces closed by 

 calcareous supporting trabeculae ; this 

 structure which is most developed near the 

 antero-ventral portion of the shell serves as 

 a float. Dorsally this portion of the shell 

 is less developed and consequently the mar- 

 gins of the shield are here much more pro- 

 nounced. Surrounding the posterior remnant 

 of the calcareous swelling is a modified forked 

 or V-shaped area (Fig. 137, g), the two ends 

 of which are directed forward; this ledge, 



vascular impressions on t be 



Pis. 136. — Median longitudinal 

 section through the shell of a 

 Belemnite, somewhat diagram- 

 matic, e.k, embryonic chamber; 

 /'/'. phragmocone ; po, proos 

 tracum ; /•, rostrum : s, siphon. 

 The dotted lines indicate the 

 anterior edge of bhe shell the 

 limits of which are at present 

 not accurately known. 



to locomotion. P " arr0W ' an ada Ptatiou favourable 



U 



