HOLOTHURIOIDEA 411 



known from Norway (Molde), the Faroe Islands, and Atlantic 

 coasts of France. Bathy metrical distribution ca. 30-200 m. 



2. Thyonidium Diiben and Koren. 

 (Syn. Orcula Troschel.) 



Tentacles 20, arranged in two circles, the outer consisting of 

 5 pairs of larger, the inner circle of 5 pairs of smaller, tentacles. 

 Tube-feet spread all over the body, except in the young specimens.^ 

 Calcareous ring without long, articulated, posterior prolongations. 

 Deposits scarce (except in young) or absent ; they have the shape 

 of tables. 



Two species known from the British (and European) seas. 



Key to the British [and European) species of Thyonidium. 



Tentacles of each pair in the inner circle placed close together. Body 

 wall thin, transparent. Tube-feet large, relatively few 



1. Th. pellucidum (Flem.) 

 Tentacles of each pair of inner circle distinctly separate. Body 



wall thick, not transparent. Tube-feet very numerous, fine 



2. Th. commune (Forbes). 



1. Thyonidium 2^dlucidum (Fleming). 

 (Fig. 248.) 



(Syn. Thyonidium hyalinum Forbes ; 

 PhyUophorus pejlucidus (Flem.), Lud- 

 wig. Bell, etc. ; Orcula Barthii 

 Troschel.) 



Body fusiform, scarcely attenuating 

 towards the ends. The tentacles of each 

 pair in the inner circle placed close to- 

 gether, so that w hen they are contracted 



thev appear much as if they are united ^^^- -^S- — Thyonidium 



. ",, , -n. 1 11 ^1 • 1 T pellucidum, with ten- 



at the base. Body wall thm, nyalme, tacles retracted. 



rather transparent. Tube-feet large, rela- Slightly reduced in 



tively few. Deposits tables with a low f^uilr'^^"'"'''''^'' 



spire and rather large disk with serrate 



edge (Fig. 249) ; in the tentacles larger, fenestrated plates 

 (Fig. 247, 3). They are fairly numerous in young specimens, 

 but they disappear with age and are then found only in the 



^ Such young specimens, with the tube-feet confined to the ambulacra, 

 must be distinguished from Pseudocucumis by their calcareous deposits 

 and the calcareous ring. 



