GENUS PiECILASMA. 101 



carina-tergal margin ; the internal basal rim generally is 

 well developed, sometimes with, and sometimes without 

 internal teeth beneath the umbones. In P. eburnea, and 

 sometimes in P. crassa, there is a line of apparent fissure, 

 and in P.fssa of actual disseverment, running from the 

 umbo to the apex of each scutum, nearly in the line in 

 which a ridge extends in Lepas • the primordial valves 

 of the scuta in these three species, are seated at the 

 basal angles of the lateral and larger segments. The 

 positions of the primordial valves, and the direction of 

 growth in the calcified valves, are, in all the species, the 

 same as in Lepas. In several of the species attached to 

 Crustacea, the two scuta are unequally convex, which is 

 caused, as was pointed out to me by Mr. Gray, by that 

 valve which lies close and nearly parallel to the body of 

 the crab, being least developed. The Terga are either 

 quite absent, or rudimentary as in P. crassa, or pretty 

 well developed as in the other species ■ the occludent 

 margin is single, and not double as generally in Lepas ; 

 the basal angle is either pointed or truncated. The Carina 

 varies considerably in shape, but never extends up between 

 the terga, nor ends downwards in a fork ; in the first 

 two species it is truncated; in the others, it terminates 

 in a deeply-imbedded oblong disc, which in P. eburnea 

 seems almost entirely (but of course not quite) to sepa- 

 rate the inside of the capitulum from the peduncle ; 

 a similar separation is effected in P.fssa, where the 

 imbedded disc is small, by two large teeth on the 

 internal basal rims of the two scuta. The carina is 

 always narrow, and either solid internally or very slightly 

 concave. 



Peduncle, is very short and narrow ; the membrane is 

 generally ringed with thicker, yellower portions, and often 

 bears very minute spines. 



Size. — All the species are small, with a capitulum not 

 exceeding half an inch in length. 



Filamentary Appendages. — None. 



MoutJi. — Labrum generally considerably bullate in 



