290 



CEPHALOPODA. 



the posterior part of which is somewhat raised and arched forward, forms 

 a cavity which in some cases (S. officinalis) is shallow, but in other species 

 (S. aculeata, Fig. 137) tolerably deep. 



These parts of the Sepia shell have been homologised with those of the 

 Belemnite shell in the following way. The spine, together with the outer 

 calcareous plate, have been thought to correspond to the rostrum of the 

 Belemnite (cf. Figs. 136 and 137) which is continued far on to the actual 

 shell, almost as if the proostracum of the Belemnite were covered by an 

 external calcareous investment connected with the rostrum, as perhaps may 

 actually be the case. A similar condition is found in Spirulirostra, in which 

 the rostrum embraces a large part of the shell (Fig. 135, r). 



The actual shell of the Belemnite, i.e., the 

 phragmocone and proostracum, corresponds 

 to the two inner layers of the shield in Sepia, 

 the prominence and the fork (Figs. 136-138, 

 w and 0). While the two outer layers are to 

 be regarded as the wall of the shell (ostracum), 

 the lamellae of the prominence may perhaps 

 be considered as partition-walls between the 

 chambers. These lamellae end posteriorly 

 in free edges (Figs. 137 and 138) ; laterally, 

 however, they are continued into corres- 

 ponding lamellae of the forked ledge. As 

 this latter rises anteriorly over the posterior" 

 end of the prominence and forms a rather 

 deep cavity posteriorly, a structure arises 

 which actually resembles the phragmocone 

 of the Belemnite. If the comparison is 

 carried further, the wide aperture of the 

 posteriorly directed partition-walls of the 

 chambers might be compared to the siphonal 

 spaces in the Ammonites, which, however, 

 have become very wide and in which a con- 

 siderable part of the body lies. 



This view seems to be supported by the 

 condition of a fossil form (Belosepia) the shell 

 of which bears a general resemblance to that 

 of Sepia but still shows the phragmocone 

 fairly distinctly (Fig. 138 B and C). In place of the siphon there is, in this 

 form, a wide cavity (Fig. 138 B), and this may be regarded as marking a 

 transition to the condition of Sepia. The rostrum in these forms resembles 

 that of Sepia, but is still more strongly developed (Fig. 138 A and B).* 



If the lamellae are regarded as partition-walls of the chambers, it appears 

 remarkable that these latter extend anteriorly, that is, that the proostracum 

 thus completely disappears, for the whole prominence would then correspond 

 to the phragmocone. 



* We refrain from comparing the anterior part of the shell (of which usually 

 only the hind or dorsal end is retained) with the shell of Sepia, but it seems 

 possible that here also resemblance can be found. 



FIG. 137. Shell of Sepia aculeata, 

 seen from the ventral surface 

 (original), d, spine ; g, forked 

 ledge ; ip, inner layer ; mp, 

 middle layer of the shield (s) ; 

 w, prominence, showing the 

 free edges of the lamellae. 



