1 82 MANUAL OF ZOOLOGY. 



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 

 (Trochocystites and Eocystites) which have been discovered in 

 the primordial zone of North America. 



4. ASTEROIDEA. These have a very long range in time, 

 extending from the Lower Silurian period up to the present 

 day. In the Silurian Rocks the genera Pa/ceaster, Stenaster, 

 Pal&odiscus, 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 Ur aster, Luidia, Astropecten, 

 Plumaster, and Goniaster, some of which have survived to the 

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

 Rocks, the genera Greasier, Goniodiscus, and Astrogonium being 

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

 fishes are known to occur, but Goniaster and Astropecten are 

 represented in the London Clay (Eocene). 



5. OPHIUROIDEA. The " brittle-stars " are represented in the 

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

 Oolitic, Cretaceous, and Tertiary Rocks several genera of 

 Ophiuroidea are known ; some being extinct, whilst others (such 

 as Ophioderma, 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 Perischoechinidce, 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 calcareous 

 plates, divided into five ambulacral and five inter-ambula- 

 cral 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 

 more rows of plates, and are surmounted dorsally by the 

 ovarian plates. The two genera, Archceocidaris and Pal&chinus, 



