1918 Bush: on Echinoderms 35 



bearing 3 equal spines forming longitudinal rows along the sides of the 

 rays. Disk usually gray ; rays white. — Not numerous, in mud at tide line. 



ORDER CLADOPHIURA Gorgon-heads 



Rays unbranched or repeatedly branched ; those with unbranched rays 

 having at the inner angle of each lower interradial space a mouth-shield, 

 1 mouth-shield serving as a madreporite ; those with branched rays having 

 no mouth-shields but with 1 to 5 madreporites in the interradial spaces. 

 Entire surface covered with a granular deposit in a thick integument. Am- 

 bulacral ossicles articulating by means of hour-glass-shaped surfaces ; 

 lateral spines none; pedicellariae-like processes sometimes present. 



1. Gorgonocephalus eucnemis Miiller & Troschel. Basket-star 



Rays 5, movable in a vertical plane and toward mouth, repeatedly 

 divided into 2 equal branches, the first division near the base ; outer branches 

 very numerous and slender. Disk medium, flattened, marked by raised lines 

 extending from each side of the bases of the rays almost to the center. 

 Upper plates of rays indistinct, covered with a granular membrane ; lower 

 plates extending to the mouth, each bearing 6 equal spines, arranged 3 on 

 each side and forming ventral marginal rows. Spines on upper surface 

 of disk and rays none, on mouth-plates many. — Numerous, in deep water. 



Class Echinoidea f 



KEY TO ORDERS 



A. Oval; anus apical. Centrechinoida (p. 35) 



AA. Disk-like; anus not apical. Clypeastroida (p. 36) 



ORDER CENTRECHINOIDA Sea Urchins 



Genus Strongylocentrotus 



Size varying, about half as thick as broad ; spines of 2 or 3 lengths ; 

 apical system with many plates; globiferous pedicellariae with end tooth 

 but without lateral teeth. 



key to species 

 A. Mature test 12.5 — 15 cm. in diameter; color purple or red. 



1. S. franciscanus 

 AA. Mature test 6-10 cm. in diameter. 



B. Color greenish; spines sharp. 2. S. drobachiensis 



BB. Color purple; spines blunt. S. S. purpuraius 



1. Strongylocentrotus franciscanus A. Agassiz. Large purple urchin 



Very large, purple but varying in shade ; spines many, i to 60 mm. 



in length, delicately fluted, pointed at tips; test large, 12.5-16 cm. in 



diameter ; apical system prominent ; genital plates larger than radials ; am- 



