Mr. J. S, Miller on the Genus Antinocamax. 65 



forming, in a transverse section, an elegant radiated ring around it. These 

 laminaB are united together by longitudinal, circular, and horizontal laminae, 

 giving this part some resemblance to the organization of a Madrepore. The 

 envelope of this organization externally, as stated before, is longitudinally 

 striated or tuberculated. In the fossil specimens, the spines always show an 

 uniform substance of carbonate of lime, having a regular fracture with the 

 angles of the primitive rhomboid ; and it is not a little remarkable, that in all 

 the calcareous fossils belonging to the Radiata, whether Crinoidea, Asteriae, or 

 Echini, the state is similar ; they are always composed of a crystalline carbo- 

 nate of lime. Only in one instance have I seen a spine imbedded in flint, with 

 the central portion penetrated by the siliceous fluid and changed into silex, 

 whilst the radiating portion was decomposed, thus leaving a space between the 

 siliceous nucleus and the impression of the exterior surface in the flint. 



It is unnecessary here to enter into details of the mode in which Echinital 

 spines grow or increase in size ; — this I reserve for some general views on 

 the organization of the Echinus. But their structure, as far as detailed here, 

 proves evidently that it differs materially from that of the Belemnite, and that 

 it differs equally from that of the Actinocamax, as the following statement will 

 show. 



The Actinocamax verus is, like the guard of the Belemnite, formed of su- 

 perposed laminae of fibrous crystals agglutinated and covered by a thin nacreous 

 investment, nearly corresponding in colour and transparency to it. It loses 

 also its colour and becomes of an opaque white on decomposition, like the 

 guard of the Belemnite under the same circumstances. This has enabled me 

 to ascertain (since in both instances the nacreous matter first yields to decom- 

 position) that in the Actinocamax, as in the Belemnite, an axis of nacreous 

 matter has existed, which by its decomposition has left a central tubular cavity 

 exactly resembling that formerly noticed in the Belemnites electricus. The 

 submamillar apex of Actinocamax verus resembles that of the last-cited Be- 

 lemnite, and the fibrous crystals assuming in both instances a transverse direc- 

 tion conformable to the curvature of the laminae, render the apex equally liable 

 to decomposition, and to drop out in a conical form. The Actinocamax on 

 holding it against the light shows that, like the belemnitic guard, it is formed 

 of two longitudinal portions, uniting by irregular edges, and separating easily 

 on fracture; thereby demonstrating that in both instances the nacreous cement 

 is less firm than the body itself. 



The lower end shows the truncated edges of all the laminae forming the club- 

 like concretions of the Actinocamax; but, contrary to what we observe in the 

 Belemnite, the exterior laminae do not project in regular succession above the 



VOL. II. — SECOND SERIES. K 



