ANNULOIDA : ECHINODERMATA. l8l 



DISTRIBUTION OF ECHINODERMATA IN TIME. Numerous 

 remains of Echinodermata occur in most sedimentary rocks, 

 beginning with the Upper Cambrian Rocks, and extending up 

 to the recent period. The two orders Cystoidea and Blastoidea, 

 which are the most lowly organised of the entire class, are 

 exclusively Palaeozoic ; and the Crinoidea are mostly referable 

 to the same epoch. The more highly organised Asteroidea 

 and Ophiuroidea commenced to be represented in the Silurian 

 period j but the Echinoidea, with few exceptions, have no 

 representative earlier than the Carboniferous Rocks. The 

 following exhibits the geological distribution of the different 

 orders of the Echinodermata in somewhat greater detail : 



1. CRINOIDEA.*- The Crinoidea attained their maximum in 

 the Palaeozoic period, from which time they have gradually 

 diminished down to the present day. As has already been 

 described, the Palaeozoic Crinoidea differ in some important 

 particulars from those which succeeded them. The order is 

 well represented in the Silurian, Devonian, and Carboniferous 

 Rocks, but especially in the latter ; many Carboniferous lime- 

 stones (crinoidal limestones and entrochal marbles) being 

 almost entirely made up of the columns and separate joints of 

 Crinoids. In the Secondary Rocks Crinoids are still abundant. 

 In the Trias the beautiful " Stone-lily " (Encrinus liliiformis) is 

 peculiar to its middle division (Muschelkalk). In the Juras- 

 sic period occur many species of Apiocrinus (Pearencrinite), 

 Periiacrinus, and Extracrinus. The Chalk also abounds in 

 Crinoids, amongst which is a remarkable unattached form (the 

 Tortoise-encrinite or Marsupites). 



Of the non-pediculate Crinoidea, which are a decided advance 

 upon the stalked forms, there are few traces ; but remains 

 of Comatula have been discovered in the lithographic slate of 

 Solenhofen (Oolite) and in the Chalk, and the genus Saccosoma 

 is Oolitic. 



2. BLASTOIDEA. The Blastuidea, or Pentremites, are entirely 

 Palaeozoic, and attain their maximum in the Carboniferous 

 Rocks, some beds of which in America are known as the 



* As regards the calyx of the fossil Crinoidea, the following terms are 

 employed to designate its different parts. The base of the cup, or calyx, 

 is termed the "pelvis," and it is made up of five, four, or sometimes three 

 plates, which are termed the "basals." To the "basals" succeed two or 

 three rows of plates, which are termed respectively the "primary radials," 

 "secondary radials," and "tertiary radials," according to their distance 

 from the basals. The axillary radials, which are the furthest removed, 

 give origin to the arms, and are occasionally called the " scapulae ' (for 

 this reason), whilst the primary and secondary radials are. called the 

 "costse." 



