406 ECHIN0DERM8 OF THE BRITISH ISLES 



the serial arrangement usually indistinct (except in young speci- 

 mens, where they are restricted to the radii, forming double 

 series). Radialia in the calcareous ring usually with a double 

 posterior prolongation (Fig. 211, 1). Calcareous deposits of 

 various forms, but not large, spinous scales, sometimes absent ; 

 often 5 prominent spicules round the anal opening (" anal 

 teeth "). 



Three species of this genus have been recorded from the 

 British seas, but two more are rather likely to occur there also, 

 namely Thy one roscovita Herouard, known from the French 

 coasts, from the Channel to Arcachon (littoral to 40 m.) ; and 

 Th. serrifera Ostergren, only known from the Trondhjemfjord, 

 200-500 m. 



Key to the species of Thyone known from or likely 

 to occur in the British seas. 



1. Calcareous deposits developed in the whole of the skin . 2 

 Calcareous deposits entirely absent from the skin or developed 



onh^ in the posterior end of t'le body .... 4 



2. Deposits with a spire, composed of 2 rods 



1. Th.fusus (O. F. MiiU.) 

 Deposits without a spire ....... 3 



3. Deposits with smooth holes ; body with a long, thin posterior 



prolongation, like a tail . 2. Th. raphanus Diib. and Kor. 



Deposits with serrate holes (Fig. 245, 2) ; no tail -like posterior 



prolongation of body .... Th. serrifera Ostergr. 



4. Deposits entirely absent, except the terminal disk of the tube- 



feet and the anal teeth ; also the 10 tube -feet surrounding 

 the anal opening contain some simple rods. Skin thick, not 

 transparent, rosy grey .... Th. roscovita Her. 

 Deposits present in the posterior part of body ; skin thin, trans- 

 parent, rosy .... 3. Th. inermis Heller 



1. Thyone fusus (0. Fr. Mtiller). (Fig. 244.) 



(^yn. Thyone jmpillosa (Miill.) Forbes ; Th. jlexus Hodge ; 

 Th. subvillosa Herouard ; Th. gadeana Perrier.) 



Body attenuating towards both ends, but the posterior end 

 not produced into a long, tail-like prolongation. The tube-feet 

 may show a more or less distinct serial arrangement. Deposits 

 (Fig. 244) not very numerous, small, smooth tables with usually 

 4 holes and a spire formed by 2 rods which unite at their 

 upper end, where they are provided with a few thorns. Some- 

 times the disk may be more irregular, with some outer holes. 

 The tables are found also in the tube-feet. Colour in life whitish 

 or rosy. Grows to a length of ca. 20 cm. 



