COMMON PEA-CRAB. 123 



of the abdomen is not at all at variance with it, as in many 

 species this part is very similar in the young of the two 

 sexes. 



It is very remarkable that Leach should have failed to 

 detect the male and female of this very common species as 

 being specifically identical. They are frequently found 

 together, and yet he describes the female as one species, 

 P. Pisum, avowing his ignorance of the male, and the male 

 as another, P. varians, acknowledging himself similarly 

 unacquainted with the female, " unless she be P, Pisum." 

 After a careful examination of the subject, I have come to 

 the conclusion that the first four species of Leach are all 

 to be referred to one ; an opinion in conformation with 

 that of Dr. Edwards. 



This species of Pinnotheres is very commonly found in 

 the common mussel, Mytilus edulis, on many parts of our 

 coast ; and especially in those which are found in rather 

 deep water. On one occasion I dredged great numbers 

 of these Mollusca on the coast of Dorset, and found 

 by far the greater number of them with one or two of 

 these little soft-bodied crabs within their shells ; for the 

 females are much more common than the males. The 

 latter sex I have occasionally taken apart from the mussel- 

 shells, the former never. They also inhabit the shells of 

 Modiolus vulgaris, and occasionally also the common cockle, 

 Cardium edule, in which I have now and then found them, 

 as well as very rarely in the oyster, in which Mr. Ball also 

 states that he has taken them. The following account of 

 some circumstances respecting this crab is extracted from 

 my friend Mr. W. Thompson's observations on the Crus- 

 tacea of Ireland,* and is too interesting to admit of being 

 curtailed. 



* " Ann. and Mag. Nat. Hist." vol. x. p. 284. 



