388 Dr. W. B. Carpenter on the Minute Structwe of 



disposed with tolerable regularity, so as to give it the appearance 

 of a cullender. These plates are connected with each other by 

 little vertical pillars, which pass from surface to surface in vast 

 numbers : hence, when a section is made perpendicularjy to the 

 surface of the plates, a kind of network is seen, formed by the 

 cut edges of the plates and by the pillars. In the shells of Echi- 

 nus, Cidaris, Spatangus, &c. these plates run nearly parallel to 

 the surface, and are of course nearly plane ; whilst the pillars are 

 perpendicular to these, so that the interstices of the network are 

 nearly square: but in the spines of Cidaris, Echinus, &c. the 

 plates are disposed in a cylindrical manner round the central 

 axis, and the network which is displayed by a transverse section 

 is arranged in concentric rings. The pattern of this network in 

 many spines is of the most wonderful beauty and elaborateness ; 

 and, as far as I have yet examined, it is so regular in the same 

 species and so diiFerent in others, that I am inclined to believe 

 that it will become most valuable in the distinction of species. 



A similar cylindrical arrangement, producing the same effect, 

 is seen in the stem and arms of the recent Pentacrinus, thin sec- 

 tions of which are extremely beautiful objects for the microscope. 



Contrary to the expectation of some of my mineralogical 

 friends, I have been able to discover the same structure in fossil 

 specimens, in which the crystallization was so complete, that 

 they could not be cut without great difficulty, on account 

 of their tendency to cleavage. This has been the case with the 

 Crinoidea in general; a definite structure being presented by 

 nearly all the specimens that I have yet examined, though it has 

 been more evident in some than in others. The structure of the 

 round-stemmed species is usually very simple ; a transverse sec- 

 tion exhibiting several concentric rings or layers, like those of 

 exogenous wood, with no variety in the pattern of the reticu- 

 lation. But in the Pentacrinites Briar eus the structure is ex- 

 tremely complex. From the centre there proceed five medullary 

 rays (if so they might be designated), from the edges of each of 

 which are given off numerous side-branches; and the reticula- 

 tions of the central portion and its prolongations are very distinct 

 from those of the periphery. Now in the recent Pentac7'inus Caput 

 Medusa the pattern is simpler ; for though the medullary rays 

 exist, they do not send out lateral branches, and their structure 

 is not so different from that of the portion that sm-rounds them. 

 In a fossil species, supposed by Mr. Miller to be identical with 

 the recent one, 1 have found the structure of the stem to be in- 

 termediate between that of the Pent. Briareus and that of the 

 Pent. Caput Medusce; the medullary rays sending out short prolon- 

 gations, which soon lose themselves in the surrounding structure. 

 I cannot but suppose, therefore, that analogous differences might 



