642 ECHINODERMATA 



FAMILY CIDABIDAE. 



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



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

 interambulacral areas 3 times as broad as the ambulacral; 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 : 



Oj Test more or less flexible 1. ECHINOTHUBIIDAE 



o. Test rigid. 



6 t Large spines hollow 2. CENTBECHINIDAE 



6 a Spines solid. 



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



c a Periproct with many plates. 



dt Ambulacral plates with 3 pairs of pores 4. ECHINIDAE 



d t More than 3 pairs of pores 5. STBONQYLOCENTBOTIDAE 



FAMILY 1. ECHINOTHUBIIDAE. 



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

 other; peristome with ambulacral 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. CENTBECHINIDAE. 



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

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

 they are attached with a central perforation; aboral ambulacral 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. 



