156 MANUAL OF ZOOLOGY. 



remains of Echinodermata occur in most sedimentary rocks, 

 beginning with the base of the Lower Silurian Rocks, and ex- 

 tending 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 ; but the Echinoidea, with a single exception, 

 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 

 Jurassic period occur many species of Apiocrinus (Pear-encri- 

 nite), Pentacrinus, and Extracrinus. The Chalk also abounds 

 in Crinoids, amongst which is a remarkable unattached form 

 (the Tortoise-en crinite 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. 



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

 Palaeozoic, and attain their maximum in the Carboniferous 

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

 Pentremite Limestone, from the abundance of these organisms. 



* 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." 



