438 ECHINODERMS OF THE BRITISH ISLES 



Greenland, ca. 10-220 ni. With the exception of Myriolrochus 

 vitreus, which reaches a lengtli of ca. 7 cm., all these species are 

 very small forms, which are easily overlooked ; they may well 

 be expected to have a much wider distribution, and thus are not 

 unlikely to occur also in the cold area of the Faroe Channel ; 

 Myriotroclius vitreus evidently is riot a cold-water species, and 

 may be expected to occur outside the cold area. 



Key to the North Atlantic genera of Myriotrochince. 



1. Wheels partly with outwardly directed thorns ... 2 

 Wheels only with large, inwardly projecting teeth (Fig. 269) 



Myriotrochus Steenstr. 



2. Wheels numerous, in several layers, of one sort only, with numerous 



short pointed knobs on the rim, pointing in various directions 

 (Theel, Sur quelques Holoth. des mers de la Nouv. Zemble, 1877, 



p. 11, Phil.) Trochodertna Theel 



Wheels not numerous, of two sorts, some only with inwardly 

 projecting teeth, others with longer, outwardly directed thorns, 

 alternating with the spokes {Holoth. Norwegian North Sea 

 Exp., p. 35, Pis. V.-VI.) . Acanthotrochus Dan. and Kor. 



The two Myriotrochus species are thus distinguished : 



Tentacles 12. Wheels very few% found only on the dorsal side in 

 the anterior and posterior end of the body (" Echino- 

 dermer ", Danmark's Fauna, p. 256) . M. vitreus (M. Sarsj 



Tentacles 10. Wheels very numerous, in several layers (Oster- 

 gren, Zur Kenntniss d. skandinav. u. arkt. Synaptiden, 1905, 

 p. elix) ...... M. Theeli Ostergr. 



Fig. 269. — Wheel of Myriotrochus vitreus. x 290. 

 (From Damnark's Fauna.) 



