224 MANUAL OF ZOOLOGY. 



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 some anomalous genera, of which the 

 best known is Protaster. In the Triassic, Oolitic, Cretaceous, 

 and Tertiary rocks various genera of Ophiuroidea are known ; 

 some being extinct, whilst others 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. 



Of the Palaeozoic Urchins or Perischoechinidcz, the two most 

 abundant genera are Archceotidaris and Palachinus, both of 

 which are principally Carboniferous, though the latter occurs 

 in the Upper Silurian. Melonites and Oligoporus are exclusively 

 Carboniferous ; and Lepidechinus and Eocidaris are principally 

 so, though both commence their existence in the Devonian. 



The Secondary and Tertiary Echinoidea resemble those now 

 living in being composed of not more than twenty rows of 

 calcareous plates. The Oolitic and Cretaceous rocks are 

 especially rich in forms belonging to this order, many genera 

 being peculiar ; but the number of forms is too great to permit 

 of any selection. 



It may be mentioned, however, that the singular genus 

 Eckinothuria, with its flexible test, the predecessor of the living 

 Asthenosoma and Phormosoma, is found in the Chalk. 



7. HOLOTHUROIDEA. This order, comprising, as it does, 

 soft-bodied animals, has left but few traces of its existence in 

 past time. The calcareous integumentary plates and spicules 

 of Sea-cucumbers are, however, by no means absolutely un- 

 known in the fossil condition, ranging from the Carboniferous 

 onwards. The shield of Psolus has also been detected in 

 Post-tertiary deposits in Bute. 



LITERATURE. 

 GENERAL WORKS. 



1. " Klassen und Ordnungen des Thier-Reichs," vol. ii. ' Strahlen- 



thiere.' Bronn. 1860. 



2. "A History of British Star-fishes and other Animals of the Class 



Echinodermata. " Edward Forbes. 1841. 



3. " Grundriss der Vergleichenden Anatomic." Gegenbaur. 1874. 



