30 



ECHINOZOA. 



central type. In this genus (fig. 273) the body is five-armed, the 

 disc being very small; and the ambulacral grooves on the under 

 side of the arms are furnished with two rows of alternately placed 

 ambulacral ossicles, bounded on each side by a row of " adambu- 

 lacral plates," which are, in turn, bordered by a series of large " mar- 



Fig. 273. — Palceaster eucharis, Devonian (after Hall). A, Under side of a specimen, four of the 

 arms being cut short ; B, Upper side of the same. «, Ambulacral ossicles, lying in the ambulacral 

 grooves ; b, Adambulacral plates ; m, Marginal plates ; o, One of the oral plates ; t, Madrepori- 

 form tubercle. 



ginal" plates. On the dorsal surface are three or more rows of 

 plates which are united by intermediate ossicles, and do not appear 

 to be separated by intervening pores. The genus Palceaster com- 

 prises some species of considerable size, and ranges from the Ordo- 

 vician to the Carboniferous. The Ordovician genus Urasterella 



Fig. 274. — A, Palasterina primceva, Silurian; b, Palceaster Ruthveni, Silurian; 

 c, Palceocoma Colvini, Silurian. (After Salter.) 



( = Stenaster) is in many respects like Palceaster, but the ambulacral 

 grooves are bordered by a row of adambulacral plates, without a 

 second row of marginal plates. Petraster, also Ordovician, has an 

 incomplete series of disc-plates between the adambulacral and mar- 



