250 



ASTEROIDEA AND OPHIUROIDEA. 



a row of adambulacral plates only, without a second row of 

 marginal plates. Petraster, also Silurian, has an incomplete 

 series of disc-plates between the adambulacral and marginal 

 rows of plates, but is otherwise almost identical with Palce- 



Fig. 142. Palceaster eucharis, Devonian (after Hall). A, Under side of a specimen, four of 

 the arms being cut short ; B, Upper side of the same, a, Ambulaeral ossicles, lying in the 

 ambulacral grooves ; &, Adambulaeral plates ; m, Marginal plates ; o, One of the oral plates ; 

 t, Madreporiform tubercle. 



aster ; whilst the Silurian and Upper Cambrian Palasterina 

 (fig. 143, A) has the disc still more extensively developed, 

 and is further distinguished by the fact that the plates of the 

 adambulacral series, which are placed at the angles of the 



Fig. 143. Silurian Star-fishes. A, Palasterina primceva, Upper Silurian ; B, Palceaster Ruth- 

 veni, Upper Silurian ; c, Palceocoma Colvini, Upper Silurian. (After Salter.) 



oral aperture, are large and triangular. The genera above 

 mentioned can hardly be grouped with any existing family 

 of the Asteroidea, as they possess several striking peculiar- 



