ANNULOIDA : ECHItfODERMATA. 139 



mostly referable to the same epoch. The more highly organised 

 Asteroidea and Ophiuroidea commenced to be represented in the 

 Silurian period ; but the Eehwtoidea, 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 lilii/ormis} 

 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-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. 



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. 

 They are, however, also found in the Silurian and Devonian 

 Rocks. 



3. CYSTIDEA. These, like the preceding, are entirely Palaeo- 

 zoic ; but they are, as far as is yet known, exclusively confined 

 to the Upper Cambrian and Silurian Rocks, being especially 

 characteristic of the horizon of the Bala Limestone. Forms 



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

 employed to designate its different parts. The base of the cxip, 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,' ' se- 

 condary 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 ' costte.' 



