ECHINOIDEA 341 



I. Family Urechinid.e. 



Mouth placed horizontally on the oral side of the test ; no 

 deep oral invagination. Second plate in all the interambulacra 

 single, the two plates of this number being placed one behind the 

 other (" meridoplacous "), as is the case in the posterior inter- 

 ambulacrum in the whole suborder. The pores all simple. 

 Apical system usually disconnected. A subanal fasciole usually 

 present. Very fragile deep-sea forms, usually of oval shape, 

 covered uniformly with short, simple spines. 



One genus, Plexechinus A. Agass., known from the British 

 seas ; a second genus, Ur echinus A. Agass., is very likely to occur 

 there also, the species Ur echinus naresianus A. Agass. being 

 known from the North Atlantic, S. and W. of Iceland, east of 

 N. America, the Mid- Atlantic, and off S. Africa ; further from 

 the Indian and Pacific Oceans, the species being thus of quite 

 cosmopolitan distribution. Its bathymetrical distribution is ca. 

 1450-4480 m.i 



The two genera are thus distinguished : 



Periproct in a depression above the ambitus 



1. Plexechinus A. Agass. 

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



Urechinus Wyv. Thorns. 



1. Plexechinus A. Agassiz. 



Test elongate oval, with a rather prominent "anal snout", 

 above which the periproct is situated in a more or less distinct 

 depression. A subanal fasciole present. Small deep-sea forms. 



Only one species known from the British seas (and from the 

 Atlantic in general). 



1. Plexechinus hirsutus Mrtsn. (Fig. 202.) 



The posterior end of test much higher than the anterior, the 

 posterior interambulacrum forming a prominent keel, prolonged 

 into a broad, slightly projecting anal snout, which is surrounded 

 by a fasciole. Four ambulacral plates enclosed by the fasciole. 

 Apical system disconnected, the posterior paired interambulacra 

 uniting in the middle of the test, thus separating the two posterior 

 ocular plates widely from the three anterior ones ; in accordance 

 herewith the anterior (frontal) ambulacrum is remarkably 



^ Urechinus naresianus is also recorded from a depth of 760 m., but 

 this seems not wholly reliable. For a full description and figures of this 

 species see ''Challenger'' Ech., p. 146, Pis. XXIX., XXX., XXX.a, and 

 " Ingolf' Ech., ii., p. 39, PI. VI., etc. 



