﻿STRUCTURE. 



23 



perforations with the short tubes to each constitutes what is called the siphuncle (a), which 

 is sometimes so close to the ventral side that its expansions touch the conotheca. 1 In 

 diag. 12 is seen the longitudinal section through the siphuncle, showing the retro- 

 flexions of the septa which form the siphuncle, and how this approaches and touches the 

 conotheca. The phragmocone is that of Bclemnites vulgaris, from the Upper Lias of 

 Yorkshire. Diag. 13 shows the marks left by the siphuncle on the "chambered cone," 

 which remains when the conotheca is wholly or partially removed. 



As already observed, the specimens are few in which the phragmocone and guard are 

 found together complete, or in such a state as to allow of a correct judgment of the whole 

 figure, if complete. Such specimens have been found in the Oolite of Solenhofen and in the 

 Oxford Clay near Chippenham. In the Lias of Lyme Regis and Yorkshire the structure 

 of the phragmocone in relation to the guard is sufficiently ascertained to justify a 

 restoration of the whole shell. In diag. 14 are presented three such figures, one from tin 



DIAGRAM 14. 



Lias. 



Oolite of Solenhofen (B. kas-tafus), two from the Lias, viz. B. paxillosus and XipJiote/if/tn 

 elongata. 



1 Voltz., loc. cit., pi. i, fig. 1. 



