HEMIASTERIDAE. 161 



A marginal fascicle, entirely separate from peripetalous fasciole, present Pericoxmus. 



No true marginal fasciole. 



Abactinal system ethmophract; genital pores 4; no lateroanal 



fasciole Hemiaster. 



Abactinal system ethmolytic. 



Genital pores 4; lateroanal fasciole more or less completely devel- 

 oped, rarely wanting. 

 Apex subcentral or anterior; anterior petal (III) narrow, not 



distinctly petaloid and not at all deeply sunken .... Protenaster. 

 Apex subcentral or posterior; anterior petal (III) wide, more 

 or less petaloid and deeply sunken. 

 Ambulacral plates, especially in III, low and numerous . Paraster. 

 Ambulacral plates, especially in III, high and few . . Prymnaster. 

 Genital pores 3 or 2. 



Peripetalous fasciole double, rarely triple, anteriorly; test 



high; genital pores 3 Faorina. 



Peripetalous fasciole single throughout. 



Peripetalous fasciole runs across interambulacra 1 and 4 

 at some distance from apical system, not following 

 closely the margins of petals II and IV. 



Petals II and IV divergent, straight, the anterior 



margin straight or a little convex; petals I and 



V usually two thirds as long as II and IV or 



more; ambulacrum III generally little sunken, 



and when petaloid, not broader than II or IV; 



apex subcentral. 



Ambulacrum III much like the others abactin- 



ally; all petals long and little sunken; globi- 



ferous pedicellariae terminate in 5 long, 



slender teeth Parapneustes. 



Ambulacrum III more or less markedly different 

 from the others; petals more or less sunken; 

 globiferous pedicellariae not as above. 



Lateroanal fasciole very distinct; petals II 



and IV do not reach peripetalous fasciole Tripylus. 

 Lateroanal fasciole partial,, indistinct or 

 wanting; petals II and IV terminate at 



peripetalous fasciole Abatus. 



Petals II and IV generally at first little divergent, 

 then becoming more so and at tip usually mark- 

 edly curved outward so that anterior margin is 

 distinctly convex; petals I and V less than two 

 thirds, often not one fourth of II and IV in 

 length; ambulacrum III usually somewhat pet- 

 aloid in adults, and often broader and more 

 sunken than the others. 



Test low, v.d. usually less than .60 length; geni- 

 tal pores 3. 

 Apex subcentral or somewhat posterior; 



