420 ECHINODERMS OF THE BRITISH ISLES 



largest specimens (15-18 mm. length) some few small warts may 

 be found on the oral valves. No tube-feet in the mid-ventral 

 radius, and along the edge of the sole they are few and relatively 

 large, apparently in a single series, those of the outer series being 

 very small and difficult to see. Deposits of the ventral sole 

 (Fig. 254, 2-3) delicate plates, usually provided with small, some- 

 what stalked tubercles. No cup-shaped deposits. The grains on the 

 scales represented only by small micro- 

 scopical, simple, perforate plates. Colour 

 white or yellowish. Very small form, 

 not known to exceed 18 mm. length. 



It may be sexually ripe at a size of 

 10 mm. It appears to propagate nor- 

 mally through autotomy, by transverse 

 division, each half regenerating the 

 lacking part. Development otherwise 

 unknown. 



This species was not hitherto re- 

 corded from the British seas. But a 



^'aoTlViL'"':TtFTJ. «P-«i'"<'" °f the Psolus sp. recorded by 

 Dan7nark's Fauna.) Massy from w^est of Ireland, which was 



sent me for examination by Dr. Stelfox, 

 Dublin, proved to belong to Ps. valvatus. It is very probable 

 also that several of the other specimens recorded by Bell, Kemp, 

 and Massy are Ps. valvatus, w^hich is probably fairly common 

 in deeper water off the Irish west coast. It is known elsewhere 

 only from the Norwegian coasts and West Greenland, from depths 

 of ca. 20-500 m. 



[IV. Order. Molpadonia Haeckel.] 



Body thick, sausage-shaped, with a tail-like prolongation. 

 Tube-feet absent, with the exception of 5 small papillae or groups 

 of papillae round the anus. Tentacles simply finger-shaped or 

 feather-shaped ; tentacle ampullae usually present. Respiratory 

 trees present. Retractor muscles may be present. Deposits 

 calcareous plates, often with a spire or an anchor ; often small, 

 claret-coloured or yellowish bodies (phosphate concretions) are 

 present. 



All Molpadids live more or less completely buried in the ground, 

 and feed on the bottom material, with which they fill their alimen- 

 tary canal. Nothing is known about the development of any 

 Molpadid. 



The order comprises one family. 



