CHTHAMALUS CIRRATUS. 461 



numbers varied in about these proportions ; but in one single specimen 

 the numbers were equal. Not only did the number of segments thus 

 vary in the two rami, but likewise the arrangement of the spines on 

 the segments in the anterior and longer ramus ; in some specimens the 

 spines on all the segments were arranged in a single circle, and then 

 the organ had a specially antenniformed appearance: in other speci- 

 mens, some of the lower segments (in one case thirteen in number) had 

 the spines placed in regular pairs precisely as on the posterior ramus, 

 and as on the three posterior pairs of cirri. Under the genus I have 

 pointed out the resemblance between this structure and that occurring 

 in certain Macrourous Crustaceans. The pedicel of the third cirrus 

 had its spines more crowded and irregular than on the three posterior 

 pairs of cirri. The segments in the latter vary in bearing either three 

 or four pairs of main spines. The whole dorsal surfaces of the lower 

 segments of the several posterior cirri are serrated in an upward 

 direction by short spines, but to a variable degree. 



3. CHTHAMALUS CIRRATUS. PI. 18, fig. 4tf, 4ib. 



Shell tvhite or gray : sheath and opercular valves generally 

 clothed by fimbriated membrane : tergum, with its basi-carinal 

 angle depending and pointed. 



Hah. — Peru, Chile, Chiloe, Northern Chonos Islands. Attached to littoral 

 rocks, aud sometimes to littoral shells, often mingled with Chthamalus scabrosus ; 

 Mus. Brit., Cuming, Darwin. 



General Appearance and Structure of Shell. — Shell dirty white or 

 gray : sometimes tinted pale purple within ; irregularly conical, or much 

 depressed, or cylindrical and much elongated. Generally much cor- 

 roded, sometimes well preserved and covered by membrane. Orifice 

 rather large, of variable shape. Sutures often quite obliterated. 

 Radii when developed narrow, with their sutural edges, as well as those 

 of the alse, generally very finely crenated, but to a variable degree. 

 The membrane lining the sheath and covering the opercular valves, is 

 remarkable from each zone being fimbriated ; for this expression is more 

 correct than to say that the membrane bears a row of spines, though 

 the fimbriae do approach in character to spines ; sometimes, though 

 rarely, the fimbriae are branched. The largest specimens which I have 

 seen (from Coquimbo and Valparaiso) were half an inch in basal dia- 

 meter, and some of these were so much elongated as to be one inch in 

 height. 



Scuta. — The scuta are rather narrow : they have a somewhat pecu- 

 liar appearance, from the articular furrow being wide, and from the 

 articular ridge projecting with a uniform curvature : the pit for the 

 lateral depressor muscle has some minute crests, of which I have seen 

 traces in the foregoing species. The Terga vary somewhat in shape : 



