GEOLOGICAL HISTORY OF STAR-FISHES. 485 



and Silurian. Among the genera peculiar to the Silurian, and chiefly 

 of Wenlock and Ludlow age, are Palaeodiscus, Lepidaster, Trichotaster, 

 Rhopalocoma, Bdellacoma, and Paloeocoma. The Devonian star-fishes 

 are Aspidosoma, Archasterias, HeliantJiaster, Xenaster. The Carbo- 

 niferous limestone has yielded no peculiar genera except Schoenaster, 

 and the forms named Calliaster and Gribellites. The Triassic types 

 are Pleur aster and Trichasteropsis. The Lias yields Astropeden, Gonio- 

 discus, Luidia, Solaster, and Uraster, which still survive, together with 

 Tropidaster and Plumaster ; Sphceraster is Jurassic. The living genus 

 Oreaster, common in the Chalk, commences in the Upper Oolites. 

 Coulonia is Neocomian, and the living genus, lihopia, is first found 

 in beds of that age. Arthraster is peculiar to the Chalk. Astro- 

 c/onium and Stellaster are first found in the Cretaceous Rocks, Very 

 little is known at present of Tertiary star-fishes. 1 



Ophiuroidea. The Brittle stars are a remarkable group of Echino- 

 derms, having the viscera excluded from the arms, which suggest much 

 the same relation to the Cystideans and Crinoids that ordinary star- 

 fishes exhibit to Echinoidea. They comprise two principal types : first, 

 that of Eurt/alus, and secondly, that of Ophioderma. Both these groups 

 are known in a fossil state ; the former, by Eudadia from the Ludlow 



Fig. 93. Ophioderma. 



rocks of England, and Onychaster from the Carboniferous rocks of 

 America. Professor Quenstedt has indicated a representative of the 

 group in the Lias. 



The Ophiuroid star-fishes are represented in the Primary rocks by 

 Protaster, which is of Silurian and Devonian age in Europe. Toeniaster 

 is of Cambrian age in Canada, and Eugaster is an American-Devonian 

 genus. The existing genus Ophioderma dates from the Muschelkalk, 

 and is well known in the English Lias. Aspidura is a characteristic 

 Triassic genus. The Oolites yield Ophiurella and Geocoma. Ophio- 

 glypha dates from the Lias, and is believed to include species which 

 have previously been referred to many genera. 



Crinoidea. The Crinoids, or sea-lilies, are an order of Echino- 

 derms in which a jointed column supports a more or less complex cup 

 1 See Wright in publications of the Palseontographical Society. 



