CHTHAMALUS STELLATUS. 459 



in var. depresswt, and in a cylindrical var, from La Plata, there was 

 quite a tuft of small spines above the notch. 



Cirri. — The outer surface of the pedicel of the second cirrus bears 

 a tuft of long, fine, plumose hairs : the terminal segments of the rami 

 of this cirrus sometimes (as in the La Plata specimens) support a clump 

 of coarsely pectinated spines. In specimens having six segments in 

 the shorter ramus of the second cirrus, the shorter ramus of the third 

 cirrus had fifteen segments. The two rami of the third cirrus are usually 

 equal in length and in the number of their segments ; but in the Brazilian 

 specimen there were fifteen segments in the posterior, and twenty-six 

 in the anterior ramus ; in another specimen, fixed on a tropical Perna, 

 there were in the two rami of this third cirrus eighteen and twenty- 

 four segments. In the three posterior pairs of cirri each segment 

 carries either four or five pairs of main spines : the segments vary a 

 little in the degree to which they are elongated, being most elongated 

 in the var. from La Plata, with an elongated shell. 



Varieties. — It will have been observed, that the shell, in the speci- 

 mens from several distant quarters of the world over which this species, 

 as I believe, ranges, differs considerably in external aspect : so do the 

 opercular valves ; and so do the parts of the mouth and cirri : but I 

 cannot make out that these differences are coordinated. Thus, var. 

 depressus, which is so entirely different from the others in appearance, 

 differs only internally in the presence of a tuft of fine spines above the 

 notch of the maxillae ; and this character is found in the La Plata 

 variety, which, as far as the shell and opercular valves are concerned, 

 is at the other end of the scale of variation. Again, var.fragilis, from 

 Charlestown, presents, in the animal's body, hardly any difference. The 

 Brazilian specimens, which in the shell and operculum offer only quite 

 common characters, have the remarkable peculiarity of a considerable 

 difference in the length and number of the segments in the rami of 

 the third cirrus ; they, also, have the segments of the sixth cirrus con- 

 siderably elongated, and the labrum finely toothed. Of these pecu- 

 liarities one alone, namely, the inequality in the rami of the third cirrus, 

 but in a lesser degree, is common to the specimens adhering to the 

 tropical Perna, which had a shell very unlike the Brazilian variety, but 

 which, on the other hand, differed scarcely in a single character from 

 some other specimens from an unknown tropical sea, in which the 

 rami of the third cirrus were quite equal. The La Plata specimens 

 differ most in internal characters, viz., in the tuft of fine spines above 

 the notch of the maxillae, in the coarsely pectinated spines on the tips 

 of the second pair of cirri, in having the segments of the sixth cirrus 

 much elongated, and in the apex of the branchiae being abruptly 

 pointed ; yet in the shell and operculum they were identical with cer- 

 tain Mediterranean varieties. From these several facts, I must believe 

 that all the widely distributed forms here grouped together, do really 

 belong to the same species. 



