CEPHALOrODA. 599 



cliiimbcred cone, and a filamentary process of appnrcntly the same 

 matter is continued down the centre of the guard to its apex. 



The structure of tlie spathosc hiyers of the guard is fibrous ; the 

 fibres being directed at right angles to the plane of the strata: viewed 

 in thin sections by transmitted light it bears a close resemblance to 

 that of the teeth of Pycnodont fishes ; but certain proportions of the 

 transverse fibres are apt so to intercept the light as to cause the 

 appearance of elongated triangular dark specks, with their apices di- 

 rected on one surface of the section to the periphery, and on the other 

 surface to the centre of the guard.* The microscopic structure, 

 which resembles that of the Pinna, proves this aggregation of sub- 

 transparent calcareous matter to be the effect of original formation, 

 and not, as Walsh, Parkinson, and Lamarck supposed, of infiltration 

 of mineral substance into an originally light and porous texture. A 

 section of a Belemnite, in which the chambers of the cone have been 

 filled with crystalline matter infiltrated from the water of the stratum 

 in which the dead shell was imbedded, affords a favourable oppor- 

 tunity of contrasting the crystalline condition of the infiltrated matter 

 with the organised texture of the solid guard. 



The exterior surface of this guard always exhibits, when entire, 

 traces of vascular impressions : it is sometimes granular, and presents 

 other modifications, which prove that it was covered by an organised 

 membrane in the living Cephalopod. I have occasionally seen the 

 remains of a more immediate investment by a thin friable layer of 

 calcareous matter analogous to that of the outer layer of the sheath of 

 the chambered cone, with which layer it becomes continuous. The 

 exterior of the spathose guard is generally impressed with a longitu- 

 dinal groove extending from the apex upwards, sometimes along the 

 ventral ( Gastrocceli), sometimes along the dorsal side {Notocceli). In 

 some of these the exterior of the guard is impressed with two oppo- 

 site longitudinal channels in addition to the dorsal groove : the un- 

 grooved Belemnites form the tribe called AccelL 



The septa of the chambered cone are perforated by a marginal 

 siphon, situated in most Belemnites on the side which is nearest the 

 dorsal line, or groove ; but in certain species of Belemnites, charac- 

 terised by the flattened form of the spathose guard, the siphon is on 

 the opposite side. These Belemnites, which at first sight look like 

 distorted or accidentally compressed specimens, are peculiar to certain 

 members of the greensand formation, called the " older Neocomian." 



Specimens of Belemnites have been discovered in which the spa- 

 tliose guard has been fractured during the lifetime of the animal ; 



* CCCXCVI. p. 67. pi. vii. fig. 2. 



Q Q 4 



