462 Messrs. T. and A. Scott on some 



The antennules are sparingly setiferous, and the long some- 

 what slender filament springs from the end of the fourth joint 

 (fig. 11). The antenna are slender, and the first and second 

 joints are each provided with one seta, while the last bears a 

 it-w setai at the apex (fig. 12). The mouth-organs are some- 

 what similar to those of Idya furcata^ but the posterior foot- 

 jaws have the terminal claws void of supplementary setas 

 (fig. 13). The swimming-feet are also somewhat similar to 

 those of Idya f areata, but the second joint of tlie outer branches 

 of the first pair is about twice the length of the first joint, 

 and the spine which springs from the outer distal angle of the 

 first joint is comparatively slender (fig. 14). The fifth pair 

 are also more elongate than those of Idya furcata, being fully 

 four times longer than broad (fig. 17). 



IJahitat. East Loch Tarbert, Loch Fyne. Several speci- 

 mens of this species were taken with the dredge in 5 to 6 

 fathoms water in February 1 886, but have not been previously 

 recorded. 



Bemarks. Idya longicornis is easily recognized, not only 

 by its larger size, but also and especially by the length and 

 structure of the antennules. The proportional lengths of the 

 joints of the outer branches of the first pair of swimming-feet 

 and the elongate fifth pair also serve to distinguish this from 

 any other British form of Idya. Though tliis Idya is the 

 largest form of the genus with which we are familiar, the 

 armature of the first pair of swimming-feet is not so powerful 

 as that of the first pair in Idya furcata, a much smaller 

 species. We are not altogether unfamiliar with the liability 

 of loya fxircata to variation, and also with the observations 

 of several eminent naturalists on this liability to variation in 

 Idya furcata ; but we have seen no explanation that could 

 account for the great difierences between Idya furcata and 

 the species we have now described on the principal of local 

 variation, for both forms were taken together by the dredge 

 on the same ground and at the same time. Had the two 

 forms been taken in different localities hundreds of miles 

 apart, there might have been some reasonableness in con- 

 sidering them merely as varieties of the one species ; but, 

 taken as they were, under similar conditions of time and 

 place, we can only judge of the differences between them as 

 we judge of the difierences between forms belonging to other 

 genera. 



Additional Notes. 



An interesting addition to the freshwater Crustacea of 

 Scotland has only lately been obtained, viz, Ophiocamptus 



