TETRACLITA COSTATA. .339 



The Basis is entirely membranous, in which respect this species 

 differs from all the others in the genus. 



Mouth : all the tropin are unusually hairy or spinose. The labium is 

 deeply notched and apparently destitute of teeth on the crest. The 

 palpi are club-shaped or enlarged at their extremities. The mandibles 

 have the fourth tooth rudimentary. 



In the Cirri, the second and third pairs are remarkably short and 

 blunt. In one specimen the two rami of the first cirrus had respec- 

 tively six and sixteen segments ; those of the second, six and seven ; 

 those of the third, seven and seven ; and those of the sixth cirrus 

 twenty rather elongated segments, with a small tuft of spines between 

 each main pair of spines. 



Affinities. — This species differs from all, in its membranous basis, 

 and in its transversely elongated scuta. It resembles T. costata, and 

 differs from all the other species, in its radii (when developed) being 

 square on the summit, and in being formed of tubes, — in the small- 

 ness and number of the parietal tubes, — in the absence of crests 

 on the under side of the scuta for the rostral and lateral depressor 

 muscles, — in the shortness and rounded form of the spur to the terga, 

 — and, lastly, in the club-shaped palpi and small size or absence of the 

 fourth tooth in the mandibles. T.purpurascens differs from T. costata 

 in those points, namely, in its membranous basis and transversely elon- 

 gated scuta, in which it differs from all the other species, and, moreover, 

 in its scuta not being plainly striated longitudinally, in having more ribs 

 on the external surface of the parietes of its shell, and in having only- 

 three pairs of main spines on the three posterior cirri. 



5. Tetraclita costata. PI. 11, fig. 2 a— 2 c. 



Shell depressed, whitish, generally with ten very prominent 

 longitudinal ribs : radii broad, with their summits parallel to 

 the basis : basis calcareous : scutum externally striated lon- 

 gitudinally : tergum with the spur short and rounded. 



Hab. — Philippine Archipelago, Mus. Cuming. Attached to various shells, 

 within the tidal limit. 



General Appearance. — Shell whitish, probably tinged, when alive, 

 with reddish-purple ; depressed ; surface perfectly preserved, smooth, 

 but having longitudinal very prominent ribs, almost invariably ten in 

 number ; namely, three on both the rostrum and carina, and two on 

 the two lateral compartments, with ten corresponding projections 

 round the basal margin. Orifice passing from rounded-trigonal to 

 diamond-shaped. The radii are very broad and square at the summit, 

 and extend from tip to tip of the compartments. Basal diameter of 

 largest specimen under half an inch, generally from *3 to '4 of an 

 inch. 



