312 Proceedings of the Boy at Physical Society. 



It is bilobular in form, its greatest diameter being in the 

 transverse direction. The figure given by Cavolini of the 

 parasite he described exactly corresponds with this species. 

 It varies greatly in size, and is undoubtedly the largest 

 known species of these parasites. Some of my specimens are 

 an inch in breadth. The skin, in the generality of specimens, 

 is of a brownish-yellow colour, and is minutely corrugated. 



This species has a wide geographical range. Cavolini 

 obtained his specimens from the shores of the Mediter- 

 ranean ; Rathke first met with it in the Crimea, and after- 

 wards in Norway ; Schmidt found it in great abundance at 

 Wangerooge, and he also obtained specimens on the Dalma- 

 tian coast ; Steenstrup's specimens were from the Mediter- 

 ranean and from the "Black Banks" in the North Sea. 

 From the observations of Thompson, this parasite appears 

 to be of frequent occurrence on the Irish coast. Some years 

 ago I found this species for the first time in the Firth of 

 Forth, but not nearly in such numbers as the following 

 one. 



Sacculina triangularis, n. sp., figs.. 4, 11, 14. 



Diagnosis. — Triangularis est, gregaria, raro sola ; maxima diametro 

 ab pendunculo ad posterius foramen pertinente : parasitus Cancri 

 paguri. 



This species is usually gregarious ; sometimes as many as 

 five individuals may be found huddled together and strug- 

 gling for existence. I have never found it on any other crab 

 than Cancer pagurus. Of the two species of these parasites 

 found in the Firth of Forth, this is by far the most common : 

 in some localities along the coast, to find a crab free from 

 it is the exception. 



The form of the animal is triangular. The greatest diame- 

 ter is in the longitudinal direction, — i.e., from its attachment 

 to the posterior orifice. Besides differing in its external cha- 

 racters from Sacculina carcini, it also differs from it, as al- 

 ready noticed, in the form and position of the vesicles. 



I am indebted to Professor Goodsir for a specimen of this 

 parasite found in the collection of his lamented brother, the 

 late H. D. S. Goodsir. It is a large specimen, apparently 



