164 - MANUAL OF ZOOLOGY. 



Paleozoic, 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 

 supposed to be Cystideans have been described from the Devo- 

 nian Rocks, but their true nature is doubtful. The- oldest 

 known Echinoderms are two extremely simple Cystideans 

 {Trochocysiites and Eocystites) which have been discovered in 

 the primordial zone of North America, 



4. AsTEROiDEA. — These have a very long range in time, ex- 

 tending from the Lower Silurian period up to the present day. 

 In the Silurian Rocks the genera Palceaster, Stmaster, Palceo- 

 discus, and Petraster are among the more important, the greater 

 number of forms being Upper Silurian. The next period in 

 which star-fishes abound is the Oolitic (Mesozoic) ; the more 

 important genera being Uraster, Luidia, Astropecten, Plumaiter, 

 and Goniaster, some of which have survived to the present 

 day. Many star-fishes occur, also, in the Cretaceous Rocks, 

 the genera Oreaster, Goniodiscus, and Astrogonium being among 

 the more noticeable. In the Tertiary Rocks few star-fishes 

 are known to occur, but Goniaster and Astropecten are repre- 

 sented in the London Clay (Eocene). 



5. Ophiuroidea. — ^The "brittle-stars " are represented in the 

 Silurian Rocks by the single genus Protaster. In the Oolitic, 

 Cretaceous, and Tertiary Rocks several genera of Ophiuroidea 

 are known ; some being extinct, whilst others (such as Ophio- 

 derma, Ophiolepis, and Ophiocoma) still survive at the present 

 day. 



6. EcHiNoiDEA. — This order is represented in the Palaeozoic 

 Rocks by a single aberrant family ; but it is numerously repre- 

 sented in the Mesozoic and Kainozoic periods. 



For the Palaeozoic Echinoidea the formation of a separate 

 sub-order has been proposed by Professor M'Coy under the 

 name of PerischoechinidcB, since they differ in some funda- 

 mental points from all the other known members of the 

 order. The test is composed of more than twenty rows of calca- 

 reous plates, divided into five ambulacral and five inter-ambu- 

 lacral areas. The five ambulacra are continuous from pole to 

 pole, and are surmounted dorsally by the ocular plates. The 

 five inter-ambulacra are composed, each, of three, five, or 



