﻿STRUCTURE. 



17 



Phragmocone. 



The conical cavity of the guard is often empty, but as often filled with a shell of 

 similar form, expanding gradually forward, and divided across by many shelly plates, 

 each concave outwardly, and pierced by a small pipe or siphuncle (<r) near one edge. By 

 these plates or septa the conical shell is divided into chambers, the last being very large 

 in comparison with the others, and destined to cover the breathing organs, heart, and other 

 viscera. The thin conical shell (c in the diagram) which covers these chambers is distinct 

 from the substance of the guard, and is called the ' conotheca' by Huxley. The whole 

 chambered mass with this shell is named " phragmocone" by Owen. It has been also 

 called " alveolus" but that title is better bestowed on the cavity in the guard which 

 receives the phragmocone. The sheath or guard extends forward over this conotheca, 

 but grows by degrees so thin as to become untraceable. Beyond the end where it thus 

 disappears the conotheca is further extended, and in some instances acquires so much 

 length and peculiarity of form as to require a separate designation. The most con- 

 venient, perhaps, is that proposed by Huxley, viz., "pro-ostracum," or anterior shell. In 

 some cases this extension seems to run out in one simple broad lobe — this appears to 

 happen in lias Belenmites ; in others — Oxford Clay species, for example — it forms two long, 

 narrow, parallel plates. Whatever be the form of this pro-ostracum, it is properly a dorsal 

 extension of the conotheca of the Belemnite. In the genus Xiphoteuthis it is a very 

 elongated part, larger than the guard, and united to it by a very long (generally depressed), 

 shelly, conothecal extension. 



The phragmocone is rarely found complete. The best examples to show its general 



DIAGRAM 6. 



Oolite. Oxford Clay. Lias. 



