644 ECHINODERMATA 



S. drobachiensis (0. F. Miiller). Green urchin. Diameter 4 to 8 

 cm. ; length of spines 8 to 15 mm. ; color green : New Jersey northwards, 

 very common north of Cape Cod; from shore to 650 fathoms; Europe; 

 Asia; Pacific coast. 



Orders. CLYPEASTROIDA. 



Sea-urchins in which bilateral symmetry of outer form has been 

 secondarily acquired; anus in posterior interradius at margin of test or 

 on oral surface; test more or less depressed or discoidal; mouth central, 

 with a dentary apparatus: 6 families. 



Family 1. CLYPEASTRIDAE. 



Bilaterally symmetrical sea-urchins with a thick shell covered with 

 short spines; in the center of the aboral surface is the 

 madreporite, from which radiate the five petaloid ambu- 

 lacral areas: 2 genera and 20 species. 



Clypeaster Lamarck. Shell more or less five- 

 sided, each petaloid area being wide and well marked; 

 each pair of ambulacral pores in the petals joined by a 

 Fig. 989 groove; 5 genital pores: about 20 species. 



^"(Snfir* ^' subdepressus Gray (Fig. 989). Shell thick and 



quite flat; color in life yellowish-green or purplish; 

 length 12 cm. ; width 8 cm. : from the shore to 40 fathoms ; North Caro- 

 lina to Brazil. 



Family 2. SCUTELLIDAE. 



Shell very flat, more or less circular, and covered with minute 

 spines; in the center of the aboral surface is the madreporite, from 

 which radiate the five well-marked petaloid areas: 8 genera and about 

 20 species. 



1. EcHiNARACHNius Leske. Shell a circular disc; each petal wide 

 and with open ends; 4 genital pores: 5 species. 



E. parma (Lamarck). Sand dollar (Fig. 990). Diameter 7 cm. 

 or less, the shell being slightly broader than long: Long Island Sound 

 northwards, from low-water mark to 800 fathoms; Pacific coast; 

 common. 



2. Mellita Agassiz. Test a circular disc pierced by 5 or 6 elon- 

 gated holes; each petal with closed ends; ambulacral grooves on the 

 oral side much branched; anus between the mouth and the interradial 

 hole in the shell: 4 species. 



