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64 Mr. J. S. Miller on the Genus Actinocamax. 



I have derived the name I have g-iven to these remains from axrig (radius) 

 and Kajjja}z (sudes)^ in allusion to its radiated fracture and the resemblance of its 

 form to the stake-like Echinital spines. The details necessary for the illustra- 

 tion of the genus are embodied in the observations on the following species. 



Species 1. Actinocamax tjerws. PI. IX, fig. 17. 



Spec. Char. A club-shaped spathose semitransparent horn-coloured concre- 

 tion ; base convex, obtuse, conical ; apex submamillar. Sides depressed 

 towards the lower end, showing two longitudinal, towards the apex 

 branching, impressions of blood-vessels. 

 Syn. and Ref. An Echinital spine of an uncommon character, bearing a re- 

 semblance to the Belemnite. (Parkinson's Organic Remains, vol. iii. PI. IV. 

 fig. 19.) 



Strata. Chalk, and sometimes inclosed in the flints imbedded in it. 

 Locality. Kent, Wiltshire, Sussex. 



Observations. 



Soon after my investigation of the Belemnite, a series of specimens of this 

 species came into my possession ; and I was astonished with the great resem- 

 blance they bore to the guard of the Belemnite in numerous respects, 

 although in general form slightly resembling the spines of Cidarites impe- 

 rialis. This resemblance induced Mr. Parkinson and some other writers on 

 organic remains to consider them as being true spines belonging to fossil 

 Echini. In order to examine mote fully into the foundation on which this 

 opinion rested, and guard against those deceptions into which a resemblance 

 of general form may so easily betray the inquirer in this branch of natural 

 history, I commenced by a minute examination of the internal structure of the 

 Echinital spines in my possession, as well those most nearly resembling the 

 Actinocamax, as others possessing a different form. 



All Echinital spines I have been able to examine, present externally a lon- 

 gitudinally striated or tuberculated surface. Near the base they have a more 

 or less finely striated rim, destined for the adhesion of muscular fibres, con- 

 necting the spine to the muscular epidermis covering the plates of the shell of 

 the Echinus. 



A transverse or longitudinal section of recent spines shows that their centre 

 is occupied by a minutely cellular calcareous substance, not unlike that (in mi- 

 niature) which occupies the centre of bones of Mammalia ; from this proceed 

 laminae, to the circumference, at a greater or less distance from each other. 



