DECAPODA. 
377 
before in herring fishing, I found in every one of them several of these 
crabs which had been brought up in the nets, and not one of any other 
species. 
Leach remarks that “ P. arcnatus differs from P. emarginatus only 
in the form of the anterior part of the shell, and may be no more than a 
variety of that species.” M. Edwards considers them the same. All the 
specimens preserved (about thirty) from the different localities mention- 
ed, — and there is about an equal number of both sexes ranging from a 
very small size up to that of Leach’s figure of P. emarginatus, — have 
the anterior part of the shell corresponding to that of P. arcuatus, or 
in other words, arched : — “ fronte arcuato integro ” is Leach’s descrip- 
tion of it. 
Genus Polybius. 
P. Henslowii, Leach. 
A crab of this species was obtained at Crook Haven, County Cork, in the 
month of August by Dr. Allman, who kindly sent it to me. It was remarked 
at the same time by its captor that the species appears to be “ eminently 
natatory,” and that “ the one taken was swimming with great ease near 
the surface of the water among shoals of Aculeplice .” It would appear, 
from the general work of Milne Edwards on the Crustacea, that this is 
the only species of its genus known. It was described by Leach from 
specimens taken" on the coast of Devonshire, and is given by M. Edwards 
as one of the species of La Manche, these being the only localities no- 
ticed for it in the two works. 
Genus Pinnotheres. 
P. Pisum, Leach. 
Templeton has noticed this species as “ dredged up in Belfast Lough.” 
It is commonly found in Modiolus vulgaris on the Irish coast, where it is 
of much more frequent occurrence than in the locality in which Dr. 
Leach endeavoured to ascertain the number found in a certain quantity 
of mussels. In the article Crustaceology (Edin. Ency.) it is remarked 
that — “ in one hundred of Mytilus modiolus, Dr. Leach found three of 
this species.” On opening eighteen specimens of the Modiolus vulgaris 
of various size — the produce of dredging off Bangor (Co. Down) in 
October, 1835 — I found fourteen individuals of P. Pisum, all females : in 
one shell only two of the crabs occurred. I have subsequently opened 
quantities of these Modioli with similar results as to the number of the 
Pinnotheres, but in all other instances more crabs were obtained from a 
like number of shells in consequence of more of the Modioli producing 
two of them. 
The smallest Pinnotheres I have seen was found by Mr. Hyndman in a 
living Cardium exiguum dredged by us in Strangford Lough in Oct., 
1834. It is a male : the carapace is under a line in length ; the entire 
breadth of the crab from the extremities of the outstretched legs is 3 
lines.* The Cardium is under 3 lines in length, and barely exceeds that 
admeasurement in breadth, so that the crab when in the position just 
mentioned must have on both sides touched the walls of its chosen prison. 
The Pinnotheres likewise inhabits the Cardium edule. Before me is one 
* In the Entomological Magazine, vol. iii., the Zoea of this Pinnotheres is 
described and figured by Mr. J. V. Thompson. 
