236 BALANID/E. 



and California, which, though differing greatly in colour, 

 resembled each other in their scuta having the adductor 

 ridge extremely prominent, and in having (PL 4, fig. 4 a,) 

 an almost tubular cavity for the attachment of the lateral 

 depressor muscle, — characters which at first appeared of 

 high specific value ; but I soon found other specimens from 

 Panama in which these peculiarities were barely developed. 

 I then examined a single specimen from the Philippine 

 Archipelago, resembling in external appearance one of the 

 Panama varieties, but differing in the scuta being externally 

 strongly denticulated in lines instead of being merely stri- 

 ated, — in the adductor ridge being far less prominent, — and 

 in the spur of the tergum being broader and more truncated; 

 I therefore considered this as a distinct species. I then 

 examined a single white rugged specimen from the coast of 

 Peru, which differed from the Philippine specimen in the 

 shape of the well-defined denticulations on the scuta, and in 

 some other trifling respects, and in the segments of the 

 posterior cirri bearing a greater number of spines; with con- 

 siderable doubt, I also named this as distinct. But when I 

 came to examine a large series of fossil specimens from the 

 Coralline Crag of England, from northern Italy, from Por- 

 tugal, and from the southern United States, I at once dis- 

 covered that the form of the denticuli on the scuta was a 

 quite worthless character, — that in young specimens the 

 scuta were only striated, — that the prominence of the ad- 

 ductor scutorum ridge and the depth of the cavitv for the 



O 1 «/ 



lateral depressor muscle varied much (as in the case of the 

 recent specimens), owing apparently to the varying thick- 

 ness of the valve, — that in the terga the spur varied con- 

 siderably in length and breadth, the latter character being 

 in part determined by the varying extent to which the 

 edges of the longitudinal furrow are folded in, — and lastly, 

 that in young specimens the basal end of the spur is much 

 more abruptly truncated than in the old. Hence I have 

 been compelled to throw all these forms, originally considered 

 by me as specifically distinct, into one species. I must 

 repeat that this considerable variation in the prominence of 

 the adductor ridge, and in the depth of the pit for the 

 lateral depressor muscle — the pit in some cases becoming 

 even tubular — is a very unusual circumstance. 



