44 THE VOYAGE OF II. M.S. CHALLENGER. 



Four nut of the five species known to Darwin live attached to Crustacea in the 

 European and eastern warmer temperate and tropical oceans. One of these, Pcecilasma 

 kaempferi, was found attached to Inachus kaempferi, de Haan (Japan), a crab probably 

 from deep water; another, Pcecilasma aurantivm, lives attached, to Homolacwoierii, from 

 Madeira, probably also a deep-water crab (Darwin); the third, Pcecilasma crassum, to 

 the same Homola cuvierii; the fourth, Pcecilasma Jissum, was found parasitic on a spinose 

 crab under a stone at low water ; the fifth, Pcecilasma eburneum, was found attached to 

 broken off spines of an Echinus from New Guinea. 



Of the two Challenger species, one comes from the Atlantic Ocean, the other from the 

 Pacific; both species were found in the neighbourhood of the coast, and living at a depth 

 of about 400 fathoms, attached to horny Corallines. 



Two fossil species of this genus have been described by Reuss. 1 Good figures of the 

 fossil remains are added to his description. The valve which represents the one species 

 is considered by the author to be the carina. But as it does not resemble the carina of 

 any of the known species of Pcecilasma, I think there are no reasons for accepting his 

 determination. The other species is represented by a valve which is no doubt a scutum 

 of a Lepadid. But this single valve is by no means sufficient to determine the genus. 



The determination of the seven at present known species of the genus will be found 

 easy by means of the following table : — 



Pcecilasma, Darwin. a 



Terga present. 



1. Basal angles of the terga cut off. 



(i.) Carina very narrow. 



(") Carina with a truncated and crested base, Paecilasma kaempferi, Darwin. 



(ii) Carina with a truncated base, .... Pcecilasma aurautium, Darwin, 

 (ii.) Carina widens downwards considerably (enlarged). 



(a) Carino-tergal margin of the scutum hollowed out 



downwards for the widened part of the carina, Pcecilasma carinafum, n. sp. 



(b) Carino-tergal margin rounded as usual, . . Pcecilasma gracile, n. sp. 



2. Basal angles of the terga pointed. 



(") Scutum not divided into two segments, . . . Pocilasma crassum, Gray, sp. 



('<) Scutum divided into two segments, . . . Pa-cilasma fissum, Darwin, 



Terga wanting, , , . Pwrilasma eburneum,WmAs, sp. 



Paicilasma carinatum, n. sp. (PI. I. figs. 8-10 ; PI. II. fig. 1 ; PI. VII. tigs. 6, 7). 



A alves five ; carina considerably enlarged downwards, terminating in an indistinct fork ; 

 ambonal teeth of the scutum not very strong; carino-tergal margin concave in the under- 

 most half for the reception of the widened part of the carina ; terga with the basal point 



1 CJeber fossile Lepadiden, Siteuwjsb. d. math.-natunc. CI. d. k' Ahid. d. Wise. U'ien, Bd. xlix. 1864, pp. 

 •21. r )-24<;, with 3 plates 



2 Darwin regards J'caHnsma as feminine, though the Greek word ilttopu is neuter. 



