PLEXECHINUS. 119 



the four cannot yet be asserted. The families are distinguished from each 

 other by the arrangement of the interambulacral plates about the mouth, and 

 the position of that organ. Whether the pores are simple or in pairs is also 

 given much weight but the simple condition may be a secondary character, not 

 primitive, in some specialized forms. 



In the Urechinidae itself, the position of the periproct, the form and char- 

 acter of the test and the relative size of the primordial interambulacral plates, 

 serve to separate the genera. I agree with Mortensen that the characters 

 supposed to separate Cystechinus from Urechinus are vague and unsatisfactory 

 and have abandoned the attempt to maintain the former genus. The pedicel- 

 lariae are plentiful and notably diversified in the Urechinidae, true globiferous, 

 rostrate, tridentate, ophicephalous, and triphyllous all occurring in more or 

 less variety of form. The globiferous may intergrade with the rostrate and 

 some forms of the latter are so much like tridentate that it is only a matter of 

 opinion which they shall be called. Both primary and miliary spines are more 

 or less prickly and the latter are swollen or expanded at the tip. Such calca- 

 reous spicules as occur in the tube-feet are simple, knobbed, or slightly branched 

 rods. 



Key to the Genera of Urechinidae. 



Periproct in a depression above ambitus Plexechinus. 



Periproct at or below ambitus, not in a marked depression. 



Test moderately high (v. d. = .50-.70 length), not flexible; plates around 

 mouth of moderate size; primordial interambulacrals much bigger than 



buccal plates Urechinus. 



Test very high (v. d. = .70-.90 length), thin and flexible; plates around mouth 

 very small; primordial interambulacrals no larger than some buccal plates; 

 globiferous pedicellariae have blade ending in a single hook Pilematechinus. 



Plexechinus. 



A. Agassiz, 1898. Bull. M. C. Z., 32, p. 76. 

 Type, Plexechinus cinctus A. Agassiz, 1898. hoc. cit. 



This genus includes certain small urechinids from cold water in widely 

 separated regions. The largest known specimens are less than an inch long but 

 seem to be adult. 



Key to the Species of Plexechinus. 



Genital pores 4, in 4 separate plates; subanal fasciole includes 3 pairs of interam- 

 bulacral plates nordenskioldi. 



Genital pores 2, minute in a single plate; subanal fasciole includes 4 pairs of inter- 

 ambulacral plates. 



