12() 



BRITISH BELEMNITES. 



s])acc of the guard, the dorso-ventral diameter being in that part much greater than the trans- 

 verse diameter. The general figure is that of Belemnites explanatus (PI. XXXVI, fig. 96). 



Locality. The Oxford Clay, upper part, at Cowley, near Oxford. The Calcareous 

 Grit and Coralline Oolite near Oxford. 



This variety agrees well with the description and figure of Blainville (' Mem. sur les 

 Belemnites '). D'Orbigny makes the phragmocone section to be more elliptical than it 

 usually is, and the axis less excentric than usual ; there is also something about the outlines 

 not as we commonly see them. 



Remarks on Specimens of Belemnitiis abbreviatus, var. excentricus, in the 

 Cabinet of Mr. Wetherell. 



The axial line of the guard is in many instances excavated into a canal which grows 

 narrower towards the apex. This is especially the case in specimens obtained from 

 the Drift of Finchley, near London, from which a great variety of fossils of the Oxford 

 Clay and other strata lying to the northward has been obtained by Mr. Wetherell. In 

 the large collection of that gentleman are very many excellent examples of this struc- 

 ture, and by careful study of them in comparison with other undisturbed specimens in 

 the Oxford Clay, Calcareous Grit, and Oxford Oolite, we arrive at a clear view of a very 

 curious subject, of which, at first sight, it might seem difficult to form a correct opinion. 



Fig. 88, PL XXXV, represents the surface produced by a splitting fracture through a 

 Belemnite in Mr. Wetherell's collection. Fractures of this kind are not infrequent in 

 nature, and are easily produced by intention. The surface thus presented is usually flat 

 and smooth in the ventral portion, as if a natural fissure existed there, but commonly 

 uneven and more or less hackly in the dorsal portion. The hollow left by the spherule 

 is sometimes traceable at the apex of the alveolar cavity, the phragmocone being generally 

 absent in the specimens under consideration, but not seldom the alveolar cavity contracts 

 gradually to the canal without any distinct enlargement at the alveolar apex. 



In the figure referred to the canal is seen to contract gradually until it finally dies 

 out before reaching the apex. Examined with microscopic powers of 10 and upwards, 

 tlie canal is found to be crossed by many ridges at nearly equal intervals, so as to 

 suggest the appearance of an annulated or half-chambered canal, in continuation with the 

 cavity or the spherule of the phragmocone (see fig. 91). The seeming septa of this canal 

 are found by more careful research to be the truncated edges of the successive laminae of 

 the guard (see figs, m, m"), each conspicuous lamina giving origin to one septum. This 

 appears quite certain under the lowest power of a good achromatic microscope, which 

 discloses, moreover, that the laminae thus referred to appear to be often composed of two 

 or three thinner layers, some dark, others paler, and probably more nacreous in substance. 



