334 LECTURE XXIII. 



M. Raspail ; but the instances of discovery of the guard associated with 

 the chambered cone left no room for doubting its relationship with the 

 Orthoceratites, although the precise degree of that relationship re- 

 quired much additional evidence for its determination. 



The spathose guard consists of successive layers, of which the exterior 

 ones run parallel with the outer surface, and progressively increase in 

 length as they approach it, thus forming the conical cavity for the lodge- 

 men of the chambered shell. The innermost or first-formed layers of 

 the guard are not parallel with the outer ones, but recede from them 

 at their upper extremity, where they form a point, and sometimes a 

 dilated nucleus, in contact with the apex of the chambered cone. 

 The interspace thus left between the early and the later strata of the 

 guard is occupied by the coarse calcareous matter continued from 

 the sheath of the chambered cone, and a filamentary process of 

 apparently the same matter is continued down the centre of the 

 guard to its apex. 



The structure of the spathose layers 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 

 directed sometimes to the periphery, and sometimes to the centre of 

 the guard. The microscopic structure proves this heavy spathose 

 aggregation of subtransparent 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. Here is 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 ; and you have a favourable opportunity of con- 

 trasting the crystalline condition of this infiltrated matter with the 

 organised texture of the solid guard. 



The exterior surface of this guard always exhibits, when perfect or 

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

 presents other modifications, which prove that it was covered by an or- 

 ganised 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 longi- 

 tudinal groove extending from the apex upwards. 



* No. 1056. A. 



