36 PALAEONTOLOGY. ■ 



or globular body which is sustained by the stem of the 

 crinoid, cystoid, and blastoid Echinoderms. 



After the microscopic organisms and the polypes, these 

 extinct pedunculated orders have taken the largest share in 

 modifying the composition of the earth's crust ; they may be 

 said to constitute some of the limestones of the Silurian and 

 Carboniferous periods. 



The chief characteristics of the palaeozoic Crinoidea are, 

 that the articulations which connect the columnar segments 

 radiate by simple striae diverging from the central axis, and 

 that the dorsal portion of the disc is equal with, or larger 

 than, the ventral portion. 



The palaeozoic types are succeeded by forms in which the 

 ventral portion is generally superior in size to the dorsal, 

 which serves only as a base for the support of the wide- 

 spreading rays, while, with two exceptions (Apiocrinus and 

 GnatJwcrinus), the columnar joints are secured by crenulated 

 noriform ridges on the facets of the joints. The crinoid type 

 continued to be richly represented to the time of the depo- 

 sition of the lias ; since which it has dwindled down to a 

 solitary Pentacrinus and a few other crinoids, having little 

 resemblance to the ancient forms. 



In tracing the progression of affinities through this class, 

 we may pass from the living Comahda in two directions, 

 forwards through a succession of beautifully graduated forms 

 to the Echinus and worm-like Holothurioids, and backwards 

 to the Marsupites, and polype-like Crinoids. But the series 

 is made more complete by the extinct species. The Sphsero- 

 nite, in which crinoidal arms have been observed,* and the 

 Echinocystites, Wy. T.f supply most interesting additional evi- 

 dence of the transition from the Crinoids and Cystoids to the 

 true urchins. 



* By Professor Wyville Thomson, "Ed. Phi]. Journ.," t. xiii. 

 f lb., plates 3 and 4. 



