116 



BRITISH BELEMNITES. 



of the groove (fig. 75 y' and v"). There is often a marked increase of depth and definition 

 of the groove for half the length of the guard, measuring back from the alveolar region, as 

 if in that part was a fissure (fig. 1\, v, v). A faint intimation of the groove can almost 

 always be traced to near the apex. 



Specimens occur with an external sheath of white fibrous matter, rough on the outer 

 surface (PI. XXIX, fig. 72 vi). One might fancy this to be a periostracon or capsule, 

 but it is, I believe, really a concretionary deposit. The shell is sometimes granulated 

 (PL XXIX, fig. 72 m). 



In figure 10, pi. xvi, of the ' Pal. Fran?.' the outhne of the alveolar cavity, erroneously 

 represented as somewhat transverse, should have been very nearly circular. The Oxford 

 specimens are never so much depressed in the post-alveolar region as in fig. 9 of the same plate. 



The axis of the guard of this Belemnite, in some specimens obtained from the Oxford 

 district, is hollow for a part of the length, as if the apices of the young laminae of the 

 guard were, during life, removed, so that a sort of pipe, partially interrupted at intervals 

 by the edges of these laminse, extended inwards from the perforated apex. Afterwards 

 the sheaths successively formed covered them completely, and were not perforated. In some 

 specimens (fig. 71 s', a', a") the axial canal is very narrow for a certain space above the 

 alveolar cavity, then it enlarges in a fusiform shape, and again contracts to the mere line 

 of junction of the opposite guard -fibres. This curious appearance Mall be further considered 

 in connection with B. abbreviatus. 



Another very curious fact is observed in several of these fossils. On the ventral 

 aspect, internally, are one or two cavities extended lengthways, through the substance of 

 the guard, from a little in front of the alveolar apex to a greater distance behind. An 

 explanation is found by the aid of cross sections : for these, taken a little behind the 

 alveolar apex, show the cavities in question to be formed by the peculiar inflexion of the 

 laminae of the guard on the ventral aspect. This inflexion becomes remarkable only after 

 a certain age ; thenceforward grows continually deeper and deeper, always producing a 

 groove, and sometimes l)y the contraction of this groove completely or partially enclosing 

 longitudinal canals. 



Fig. 71 5 shows the arrangement of the laminae round the axis of the guard in 

 conformity with this description. The axis is not tubular in this instance. The laminae 

 of the guard are crossed by the fibres nearly at right angles to the surface, and as this is 

 a curve of contrary flexure about the ventral aspect, the fibres assume there remarkably 

 arched directions. In these sections glisteniug dagger-shaped parts are present — they are 

 merely the obliquely truncated prismatic cells of the so-called fibrous structure. It may 

 be well to mention, that the specific gravit}- of most Belemnites (2-8) agrees with that 

 carbonate of lime called arragonite, and not with ordhiary calcite. 



The student of Homology will not fail to remark the analogy which this repetition of 

 deep folds on the ventral aspect of Belemnites sulcatus off'ers to the more regular groove 

 on the same aspect in Belemnitella. The groove of the latter group, however, is only on 



