120 GEOLOGICAL MEMOIRS. 



phragmocone, lodged in the conical cavity at the base of the 

 guard. 



3d. The conical, thin, but dense corneo-nacreous case, which 

 immediately invests the phragmocone and lines the alveolus of the 

 guard, commencing at the bottom or apex of that cavity, and con- 

 tinued beyond 'the last septum of the phragmocone to form the 

 large anterior chamber of the Belemnite, containing the ink-bag 

 and some other viscera. 



The Belemnites have been classified according to the modifi- 

 cations of the sheath, and the species described is characterised by 

 a rounded elongated conical guard, with a short terminal, ventral, 

 longitudinal impression. It is called B. Owenii Pratt, and ap- 

 proximates in general form to B. elongatus and B. longissimus 

 Miller, from the lias. The excavated part of the guard becomes 

 very thin as it expands, and its thin and brittle margin may be 

 traced nearly half way towards the base of the phragmocone, 

 which is there invested only by the thinner and more yielding 

 corneo-nacreous sheath. With respect to the guard, the most im- 

 portant additional information obtained by these specimens is an 

 account of its microscopic structure. It consists of numerous thin, 

 for the most part concentric, layers of minute prismatic trihedral 

 fibres, placed at right angles, or nearly so, to the planes of the 

 layers ; the crystalline fibres are indicated by lines, which radiate 

 from the central axis and cross the lines of growth ; the lines 

 which define the fibres run in pairs with a minutely and gently 

 undulating course, resembling the tubes of dentine, but differing 

 in the transparency of the intercepted calcareous matter, which is 

 like that in the wider spaces separating the pairs of lines. 



These differences in the intervals of the radiating fibres may 

 depend on the different parts of the prismatic fibres, divided in 

 preparing the sections made parallel to their course. 



The exterior surface of the guard of the Belemnites is minutely 

 granular, and occasionally presents faint traces of vascular im- 

 pressions, proving it to have been invested by an organised mem- 

 brane of the living Cephalopod ; and in two specimens from the 

 Oxford clay there have been detected remains of the more imme- 

 diate investment of a thin friable layer of white calcareous matter, 

 analogous to that of the outer layer of the sheath of the phragmo- 

 cone. It is only necessary to add, with reference to the spathose 

 calcareous constituent, that its microscopic structure proves it to 

 be an original formation, deposited in membranous cellular moulds 

 under the influence of the vital organising forces, and not the 

 result of post-mortem infiltration of mineral substance into an 

 originally light and porous or cellular texture. 



As respects the phragmocone and its investing sheath, it is clear 

 from the Oxford clay specimens, that the sheath is continued 

 backward to line the alveolar cavity of the guard, as well as for- 

 ward from its basal outlet to form the visceral chamber anterior to 

 the phragmocone. The phragmocone appears broader than it actually 

 was, on account of the compression which its frail walls were 



