364 



ECHINODERMATA. 



in his researches. Thus the singular fossils described by Eichwaid 

 under the name of Bolboporites can by this means be clearly deter- 

 mined to be of Echinodermal origin. These curious remains occur 

 in the Ordovician rocks of Russia and Canada, and though variable 

 in shape, have most commonly the form of inversely conical, ovoid, 

 or clavate bodies, about a quarter of an inch or less in their long 

 diameter, and composed of calcite with its characteristic cleavage. 

 The narrower end of the body is usually smooth, and the sides or 

 upper surface generally pitted with shallow depressions. Described 

 originally as corals, the microscopic appearances of thin sections 

 render it certain that the fossils placed under Bolboporites are parts 









Fig. 236. — Minute structure of Echinodermal plates. A, Part of a small plate (madreporite ?) 

 of an Echinoderm from the Carboniferous rocks of Scotland, highly magnified ; b, Part of an 

 interarnbulacral plate of Lepidocentrus, from the Devonian of the Eifel, highly magnified. 

 (Original.) 



of the exoskeleton of some Echinoderm. It is difficult to speak 

 precisely as to their nature, but they may be compared with the 

 large tubercles or swollen spines which are developed in the integu- 

 ment of certain Star-fishes {e.g., Pentaceros). 



As regards their classification, the Echinodermata may be divided 

 into seven primary groups, which are now usually regarded as 

 classes — viz., the Crinoidea, Cystoidea, Blastoidea, Ofihiuroidea, As- 

 teroidea, Echi?ioidea, and Holothuroidea. Of these, the first is to a 

 considerable extent extinct, and the two next are entirely so ; while 

 they exhibit certain structural peculiarities which separate them from 

 the other classes. More particularly, the members of these three 

 classes — viz., the Crinoids, Cystoids, and Blastoids — all possess a 

 dorsally-developed, jointed calcareous stalk, which serves to fix them 

 to foreign objects, and which may be only temporarily present. 

 From the presence of this jointed stem, these three classes are 

 grouped together in a single great division, under the name of Pel- 

 matozoa. On the other hand, the Echinoids, Asteroids, Ophiuroids, 

 and Holothuroids are devoid of this stalk at all periods of develop- 



