MOLLUSC A— THE SHELL 



69 



In Spirula (E), the shell is coiled spirally and endogastrically. The siphuncle 

 is thick, and is surrounded along its whole length by septal envelopes. The rostrum 

 is rudimentary, and there is no pro-ostracuni. 



Starting again from the Belemnites, the modification of the shell may take another 

 direction. The phragmocone may become smaller and shorter in comparison with 

 the continually lengthening pro-ostracum (e.g. Ostracotevfhis, F). The rostrum 

 also may become thinner and smaller. Finally, the shell may be reduced to a very 



Fro. 07.— AH., Diagrammatic median sections through the shells of eight extant or fossli 

 Dibranchia, from the left side. The point of the visceral dome is turned downwiirds, the posterior 

 side of the sliell is tn the left and the anterior to the right (i/. the position of the C'ephalopod body, 

 p. 30). A. Sepia; B, Belosepia (fossil); i\ Belemnite (fossil); D, Spirulirostra (fossil); ;•;, 

 Spinila; i^, Ostracoteuthis (fossil); G, Onunastrephes ; //, Loligopsis; ph, chambered shell = 

 Iihragmocone ; pr, pro-ostracuin ; r, rostrum (guard) ; s, siphuncular canal, or space which con- 

 tains the siphuncle ; 1, 2, 3, last three septa (the most recent) ; n, anterior wall of the siphuncle ; }), 

 posterior ; x, anterior edge of the first septal or siphuncular envelope = anterior or jiosterior edge of 

 the siphuncular canal. 



small hollow cone at the end of a long narrow horny lamella which coriesponds 

 with the pro-ostracum, and is called, in the extant Decapoda, the gladius or calamus 

 (or pen) {Loligo, Ommastrephes (G), Onychotcuthis). In Dosidicus, this terminal cone 

 is almost solid, and in Loligopsis (H) it is nothing more than a thickening at the 

 upper end of the gladius ; in other Dccnpoda, there is no trace of it on the gladius. 

 In the Odopoda, the shell has completely disappeared. 



Again starting from the Belemnite, the shell may develop in a third direction 

 to form the Sepia shell. The transition form is found in Belosepia (B) (Eocene), 



