CHARACTERISTIC DEEP-SEA TYPES. SEA-URCHINS. 89 



acters derived from the study of a limited number of specimens, 

 both in the fossil and recent species. In the seas of the Jura 



Fig. 348. Dorocidaris papillata. f . 



and of the chalk the Cidaridse must have been common types 

 of sea-urchins. Dorocidaris Blakei (Fig. 349), obtained by 

 the " Blake," is perhaps the most interesting of the recent Cida- 

 ridae, from the variability of its spines. Before the " Blake " 

 dredgings none were known among the recent species showing 

 any great or striking variety in the form of the radioles. With 

 the exception of some of the species of the genus Goniocidaris, 

 the radioles are characterized by their uniformity, while among 

 the fossils of the family the variation in shape and size of some 

 of the Jurassic and cretaceous species is quite remarkable. If 

 the present species had been dredged without its two or three 



