CLASS I ASTEROIDE A 251 



I'amily 6. Oreasteridae (wrongly Pentacerotidae). 

 This family includes some of the largest Eecent Starfishes, characterised by 

 a massive skeleton, with large, though sometimes concealed, marginal plates. 

 Conspicuous spines or tubercles are commonly found on the abactinal plates. 

 The type genus Oreaster M. and T. (wrongly Fentaceros) (Fig. 355), is wide- 

 spread in shallow water in the tropics, while geologically it is known at least 

 as early as the Upper Jura. Numerous species of this genus occur in the 

 Cretaceous. Arthraster Forbes, and Stauranderaster Spencer, from the British 

 '^retaceous probably belong to this family. 



Family 7. Sphaerasteridae. 

 Isolated plates, to which Quenstedt gave the name Sphaerites, from the 

 Jurassic rocks of Germany, France and Switzerland, have long puzzled 

 paleontologists, but Schöndorf has recently shown that they belong to certain 

 remarkable Starfishes, which he calls Sphaeraster, allied to the Oreasteridae. 

 In some cases (Fig. 356) the plates bear large spines, but in others they are 

 simply punctate (Fig. 357), or quite smooth. The animal was high hemi- 

 spherical in form, and the large ones were 25 cm. in diameter. They seem to 

 have been confined to Jurassic seas. 



Order 2. CRYPTOZONIA. 



Asteroidea in which the marginal plates are small and inconspicuous ; papulae 

 distrihuted on the oral surface ; amhulacral ossicles are often crowded and tube-feet 

 may he infour rows in each groove. 



Between fifteen and twenty families of cryptozonate Starfishes are now 

 recognised, but the great bulk of these are Recent forms. The order is rare in 

 the Paleozoic, and the structure of those forms which are referred to it is im- 

 perfectly known. Accordingly, their systematic position is doubtful. The 

 genus Palasterina M'Coy (Cambrian to Devonian ; Europe and North America) 

 is regarded by some writers as cryptozonate and by others as phanerozonate. 

 It is probably related to Asterina Nardo, a widespread Recent genus, which 

 Sladen considered phanerozonate, other writers to the contrary notwith- 

 standing. The genera Palaeocoma, Bdellacoma and Bhopalocoma Salter are 

 probably Cryptozonia but their family position is very doubtful. 



Lepidaster Forbes, of uncertain affinities, is an interesting Silurian genus 

 with large disk and thirteen rays. Ethendgaster Gregory, from the Carboni- 

 ferous of New South Wales, is considered by its describer as a related genus, 

 although it has only five rays and was originally regarded as a Palaeaster. 

 Medusaster Stürtz is notable for having fourteen rays, and Helianthaster Roemer 

 is another remarkable form with sixteen rays. The latter has been regarded 

 by some as a Starfish and by others as an Ophiuran ; it is probable that it 

 belongs in the Cryptozonia, but most unlikely that it is related to the Recent 

 South American Heliaster. 



It is possible that the Recent family Linckiidae isrepresented in the Devonian 

 by Roemeraster Stürtz, but the relationship is very dubious. The genera 

 Palasteriscus and Echinasterella of Stürtz from the Devonian are said to have the 

 madreporite on the oral side, which would alone render them worthy of note. 

 Loriolaster and Cheiropteraster Stürtz, also from the wonderful Bundenbach 

 slates, are possibly allied to the Recent Pterasteridae. Mesozoic and Tertiary 



