628 ECHINODERMATA 



Family 4. OREASTEKIDAE. 



Thick-bodied starfishes with small marginal plates, and a reticulate 

 arrangement (Fig. 974 B) of aboral plates: 10 genera. 



Oreaster Miiller and Troschel {Pentaceros Linck). Body very 

 regularly star-shaped, with short arms and large central disc; plates 

 with tubercles: 40 species, which are all littoral and subtropical or 

 tropical. 



0. reticulatus (L.) (Fig. 974). Diameter 30 to 40 cm.; body very 

 thick: South Carolina and southwards, in shallow water. 



B 

 A 



Fig. 974 — Oreaster reticulatus (Tennent). A, outline of body ; B, aboral surface of arm. 



Family 5. ASTEEOPIDAE. 



Starfishes whose body is covered by a thick, smooth skin, concealing 

 the underlying skeleton : 7 genera. 



Dermasterias Perrier. Body stellate and thick; no spines on the 

 marginal or aboral plates: 1 species. 



D. imbricata (Grube). Size 15 to 20 cm. : on rocks in shallow water: 

 Alaska to Monterey, California; common. 



Order 2. SPINULOSA. 



Arms usually long and without prominent marginal plates; aboral 

 body wall spiny and usually Avith a more or less reticular or imbricated 

 skeleton; ambulacral feet with sucking discs; pedicellariae rare or absent 

 and not pedunculate: 5 families. 



Key to the families of Spinulosa here described: 



c. Mouth plates small ; ambulacral groove narrow. 



bi Aboral skeleton composed of imbricating scales 1. Asterinidae 



&2 Aboral skeleton reticular 2. Echinasteridae 



Oj Mouth plates large ; ambulacral groove wide. 



fci Usually more than 5 arms 3. Solasteridae 



62 Five arms present - 4. Pterastebidae 



