﻿STRUCTURE. 



15 



DIAGRAM 3. 



The phragmocone was formed on the exterior of a secretive surface, and the fibrous sheath 

 on the interior of another secretive surface, both extended together from the germinal 

 capsule. The incremental striae are external on the phragmocone, and internal on 

 the guard. 



The guard is sometimes fusiform when young, and grows cylindrical or conical with 

 age ; but it is doubtful if any example is satisfactorily in accordance with 

 the figure of Blainville (pi. i, fig. 4), which represents a fusiform mass en- 

 closed in lamina) of growth quite parallel to it. In some cases detached 

 Belemnites assume a fusiform shape by the decay of the lamina? in the peri- 

 pheral parts about the point of the alveolus. This seems to arise from a 

 difference of composition of the laminae thereabout. It is seen very often 

 in the elongated Belemnite of the Speeton Clay, and in specimens of Belem- 

 nites quadratus. On such as these Miller commonly founded his genus 

 Actinocamax. 



The surface of the guard is sometimes marked by ramified impressions of 

 vessels, and more frequently by certain systematic grooves, which furnish 

 some of the grounds for convenient classification. 



Confining our attention at present to the guard, it may be remarked that 

 the anterior edge is seldom completely traceable ; we may, however, be sure 

 that in every case the dorsal edge and its alveolar cavity extended much 

 farther forward than the ventral edge, as represented in Diag. 1. Another 

 point of importance regards the plane of symmetry of the shell, for this always 

 passes directly through the axis or apicial line of the guard, from the middle 

 of the dorsal to the middle of the ventral face. On each side of this plane 

 (except in cases of deformity or accident) the masses and areas are equal 

 and equally disposed, and the markings of the surface are in pairs, while 

 along the middle line thus defined on the dorsal and ventral faces the markings are 

 single. Thus, three aspects of the guard always require attentive inspection and special 

 description, a circumstance rarely observed by Belemnitologists. 



The generally smooth surface of Belemnites is only broken by grooves of greater or 

 less depth, striae, and small plications, and continuous or interrupted small ridges. All 

 these correspond to peculiarities of the formative membranes, and are sufficiently charac- 

 teristic and constant to serve for more than specific distinction, as will more fully appear 

 hereafter. 



The successive laminae of the guard are not parallel, but in the greater number of cases 

 grow thicker towards the point, and remarkably thinner towards the anterior edges. 

 These laminae meet the apicial line (a) at angles usually acute, and still more acute in their 

 intersection with the conical cavity, which receives the phragmocone, so that they might 

 even appear to be parallel to it. As many as three hundred laminae of growth have been 

 counted in the cross section of the solid part of a Belemnite from the Oxford Clay, and 



The part 

 marked W is 

 often wasted 

 away in soft, 

 brittle larainse. 



