642 ECHINODERMATA 



Family CIDARIDAE. 



With the characters of the order : 10 to 12 genera. 



EuciDARis Pomel. Oral and aboral surfaces flattened; test thick; 

 interambiilacral areas 3 times as broad as the ambuiacral; primary 

 spines usually shorter than the diameter of the test, stout, fluted, and 

 granulated; between these are smaller spines: 3 species. 



E. tribuloides (Lamarck). Diameter 4 to 7 cm.; color of primary 

 spines grayish, broad and flat : South Carolina to Brazil ; common ; from 

 shore to 116 fathoms. 



Order 2. CENTRECHINOIDA. 



Sea-urchins with peristomal gills and sphaeridia; mouth central, with 

 jaws ; anus apical : 10 or 11 families. 



Key to the families of Centrechinoida here described: 



Ci Test more or less flexible 1. Echinothuriidae 



o. Test rigid. 



6i Large spines hollow 2. Centbechinidae 



fta Spines solid. 



Ci Periproct with but few, usually 4, plates 3. Abbaciidae 



c, Periproct with many plates. 



di Ambuiacral plates with 3 pairs of pores 4. EcniNiDAE 



da More than 3 pairs of pores 5. Stbongylocentrotidae 



Family 1. ECHINOTKURIIDAE. 



Test flexible, the plates being movable and overlapping one an- 

 other; peristome with ambuiacral plates and feet, but without interam- 

 bulacral plates: 6 genera. 



Phormosoma Wyville Thompson. Difference between oral and 

 aboral sides marked; primary spines on the oral side encased in a thick 

 membrane: 4 species. 



P. sigsbei (A. Agassiz). Diameter 6 to 8 cm.; spines small, reddish- 

 orange in color: West Indies, in deep water. 



Family 2. CENTRECHINIDAE. 



Sea-urchins with peristomal gills; ambuiacral areas narrow; inter- 

 ambulacral areas large; spines long and hollow; the tubercles to which 

 they are attached with a central perforation; aboral ambuiacral feet 

 without suckers: 9 genera; mostly in the Pacific and Indian Oceans; 20 

 to 25 species. 



Centrechinus Jackson (Diadema Gray). Test circular in outline, 

 somewhat flattened, about twice as wide as high; colors very dark; blue 

 eye spots often present on the genital plates and elsewhere: 6 species. 



