398 



ECHINOZOA. 



and Palastropecten\ but the still existing genus Astropecten (Asterias 

 of many authors) is represented by a single species (Stiirtz), and 

 the same genus is said to occur in the Carboniferous rocks. If some 

 doubt attaches to the Palaeozoic forms which have been referred to 

 Astropecten, unquestionable remains of species of this genus occur in 

 the Mesozoic and Tertiary rocks, the earliest appearing in the Lias. 

 The genus belongs to the group of Star-fishes (Phanerogonid) in 

 which the tube-feet are in two rows and the " papulse " confined to 

 the dorsal surface, and is recognised by its five-rayed form, and 

 flattened disc and arms, the edges of which carry a double row of 

 large "marginal plates" (figs. 269 and 275). The lower of the two 



Fig. 275. — Under surface of Astropecten Phi!lif>sii, of the natural size. 

 Jurassic. (After Wright.) 



rows of marginal plates is furnished with spines, and the whole 

 of the upper surface is covered with tubercles crowned by groups 

 of minute prickles. 



Other genera of Phanerogonia which are abundantly represented 

 in the Jurassic, Cretaceous, and Tertiary rocks, and which still sur- 

 vive, are Gom'aster, Astrogonium, Goniodiscus, and Stellaster. The 

 first-named genus comprises the so-called " Cushion-stars," and is 

 characterised by the fact that the body has the form of a pentagonal 

 disc (fig. 271), in which the arms are principally recognisable by the 

 presence of the ambulacral furrows on the inferior surface. The 

 disc is bordered by a double row of large " marginal plates," and 

 the upper and lower surfaces are covered with small, four-sided or 

 polygonal plates. Besides the above, the existing genera Pentaceros 

 (Oreaster) and Luidia appear in the Jurassic, though the occur- 

 rence of the latter is not certainly established. Among the Crypto- 



