OPHIUROIDEA. 115 



No thoroughly satisfactory classification of the Asteroidea 

 has been as yet proposed, but the following are the names of 

 the more important fossil genera, with their range in time : 



Stenaster. Lower Silurian. 



Petraster. 



Palceaster. Lower Silurian to Carboniferous. 



Palasterina. Upper Silurian. 



Pal&ocoma (Salter). 



Paltpodiscus. 



Lepidaster. 



Pleurasier. Trias. 



Uraster. Jurassic to Recent. 



Solaster. 



Astrogonium. 



Goniaster. 



Astropecten. 



Luidia. 



Arthraster. Cretaceous. 



Oreaster. 



Stellaster. Cretaceous to Recent. 

 Goniodiscus. 



ORDER III. OPHIUROIDEA. 



The Ophiuroidea are often grouped with the Asteroidea, and 

 the living members of the order are known commonly as 

 Brittle-stars and Sand-stars. They are distinguished from the 

 true Star-fishes by the fact that the ""disc" contains all the 

 internal organs of the animal ; the " arms" are not grooved in- 

 feriorly for the emission of ambulacral tube-feet ; and the mouth 

 is provided with a masticatory apparatus. The Ophiuroids are 

 very conspicuously star-shaped, and consist of a central " disc " 

 and a series of radiating " arms " (fig. 69). The " disc " is 

 truly disc-shaped, and is covered with granules, spines, or 

 scales. From the disc proceed the arms, in the form of long 

 and slender processes, which may be simple or branched, but 

 which differ from the arms of Star-fishes in not containing any 

 prolongations from the stomach, and in never having their 

 under surfaces furrowed by ambulacral grooves. The arms, 

 in fact, are special processes superadded for the purposes of 

 prehension and locomotion, and rendered necessary by the 

 fact that the ambulacral system takes no part in the function 

 of locomotion, as it does in the Star-fishes. A madreporiform 

 tubercle, however, is present, and is placed on the inferior 

 surface of the body, being commonly concealed by one of the 

 plates surrounding the mouth. The mouth, as in the Star- 

 fishes, is placed in the centre of the lower surface of the disc ; 



