DIBRANCHIATE CEPHALOPODS. 



873 



be detected. In the extinct family of the Belemnitidce, the skeleton 

 consists of a well-developed chambered cone (the " phragmacone "), 

 which is divided by arched septa into a succession of air-chambers, 

 the septa being pierced by a tube or " siphuncle." The phragmacone 

 in this family is prolonged forwards on its dorsal side into a flattened 

 "pen" (the " pro-ostracum "), while it is protected posteriorly by 

 a solid calcareous investment or sheath (the " guard "). In the 

 existing genus Spirilla the shell is internal, but is reduced to the 

 chambered "phragmacone," which is coiled into a flat spiral, the 



737: 



globular or 



and a). 



inflated "proto- 



cols of which are not in contact (fi 



The phragmacone commences in a 

 conch" or "initial chamber" 

 (fig. 805, fir), which is dis- 

 tinctly constricted off from 

 the first air-chamber, and 

 which is devoid of a cica- 

 trix. The siphuncle com- 

 mences as a cascal tube (c) 

 pushed into the protoconch ; 

 and a " prosiphon " (/) is 

 present. It will be thus seen 

 that the protoconch of the 

 Dibranchiates is essentially 

 similar to that of the Am- 

 monoids, a fact which has 

 led some naturalists to the 

 belief that the latter are truly 

 Dibranchiate, and that their 

 shell was therefore internal. 

 In all the Dibranchiates in 

 which a phragmacone is com- 

 pletely developed, the siphun- 

 cle is constantly ventral in 

 position, and in Spirilla it is consequently placed along the concave 

 side of the shell. 



As regards their classification, the Dibranchiata are divided into 

 two sub-orders, the Octofioda and the Decapoda, the former charac- 

 terised by the possession of eight arms only, while in the latter 

 there are eight comparatively short arms and two long arms or 

 " tentacles." The Decafioda constitute the most important of these 

 groups, and are divided by Fischer into the three sections of the 

 Phragmofihora, Sepiophora, and Chondrophora, in accordance with 

 the nature of the skeleton. 



As regards the distribution of the Dibranchiate Cephalopods in 

 time, no Palaeozoic types of the order have hitherto been detected. 



Fig. 805. — Longitudinal section of the shell of 

 Spirula Peroni. fir, Protoconch ; fi, Prosiphon ; c, 

 Caecal commencement of the siphuncle ; s i-, Siph- 

 uncle ; se, One of the septa : w, Ventral wall of the 

 shell. (After Munier-Chalmas.) 



