ECHINODERMA. ASTEROIDEA 111 



spines. Ad-ambulacrals boot-shaped. Disc with rounded isolated 

 plates ; on the aboral surface a central plate is surrounded by a 

 circle of five pairs of plates, and four rows of plates occur on the 

 aboral surface of each arm. Ordovician and Silurian. Ex. P. 

 primceva, Ludlow. 



Calliderma. Body flattened, pentagonal-stellate, with the 

 rays moderately long. Marginal plates large, forming a broad border 

 to the disc, covered with granules. Aboral surface of disc with small 

 plates arranged regularly. Cretaceous to present day. Ex. C. 

 smithice, Chalk. 



Metopaster. Body flattened, pentagonal in outline, the rays 

 only slightly produced. Marginal plates thick, covered with punc- 

 tations and surrounded with a depressed border. Supero-marginal 

 plates few in number, forming a broad border to the disc ; the 

 terminal pair of plates the largest. Aboral surface covered with 

 small polygonal (usually hexagonal) plates. Infero-marginal plates 

 more numerous than the supero-marginals. Plates on the oral 

 surface small, polygonal. Cretaceous. Ex. M. parkinsoni, Upper 

 Chalk. 



Mitraster. Similar to Metopaster, but rounded (or slightly 

 pentagonal) in form, with supero- marginal plates few and all of 

 equal size. Chalk. Ex. 31. hunteri. 



PalaBOCOma. Arms rather short. Ambulacral grooves narrow 

 and shallow ; ambulacral ossicles numerous and alternating on either 

 side. Ad-ambulacral plates large ; the adjoining row of plates with 

 numerous long spines. Disc with very small plates. Silurian. 

 Ex. P. marstoni, Ludlow. 



Distribution of the Aster oidea 



The Asteroidea have a wide distribution in the ocean 

 at the present day ; they are most abundant at moderate 

 depths, but also occur in abyssal regions. 



The fossil forms do not differ greatly from the modern 

 ones, but many of those found in the Paleozoic formations 



