261 Part III. — Seventeenth Annual Report 



proportional lengths of all the joints are nearly as in the annexed 

 formula — 



Number of the joints, 1 ■ 2 • 3- 4 ■ 5 ■ 6 • 7 " 8 



Proportional length of the joints, 22 • 28 • 25 • 24 • 6 ■ 9 ■ 4 • 18 



The antennae and mouth organs are similar to those of Idya furcata. 

 The first pair of thoracic feet resemble those of Idya furcata, but the seta 

 that springs from the end of the first joint of the outer branches is short 

 and straight, and none of the terminal or sub-terminal setse bear secondary 

 spine-like apical cilia so characteristic of Idya furcata and one or two 

 other members of the genus. The second joint of the inner branches is 

 proportionally stouter than the same joint in Idya furcata, and the 

 terminal claws are long and slender instead of being short and moderately 

 stout. Moreover, the spines on the inner and outer aspects of the second 

 basal joint are also small and slender (fig. 4). The second, third, and 

 fourth pairs of swimming feet are rather more slender than the same 

 appendages in Idya furcata (fig. 5). The fifth pair of feet have the 

 second joint long and slender, and the margins do not appear to be 

 ciliated ; the seta which springs from the inner angle, and also that 

 which springs from the outer angle, of the basal joint are long and slender, 

 while the terminal setse of the secondary joint are also elongate (fig. 6). 

 The abdomen is elongate, being equal to nearly two-thirds of the length 

 of the thorax ; the first and second segments appear to be, at least partly, 

 coalescent ; their combined length is equal to half the entire length of the 

 abdomen ; the last abdominal segment is very small. The caudal furca 

 are short, and about as long as broad. 



Habitat. — Loch Fyne and Firth of Clyde. Generally distributed, and 

 apparently not very rare. 



Remarks. — This distinct species of Idya appears to be unlike any 

 previously described member of this genus. The two most prominent 

 characters by which it may be distinguished from all closely allied species 

 are — (1) The armature of the first pair of thoracic feet, and especially the 

 long terminal spines of the inner branches, and (2) the long slender fifth 

 feet. Idya cluthce may by these two characters be distinguished at a 

 glance even without dissection. Like other forms of Idya, this one bears 

 a comparatively large ovisac. Both males and females have been 

 obtained, and both are equally distinct. Hitherto this species has 

 occurred only in moderately deep water. 



Monst villa dance (1), Claparede. 



Several specimens of Monstrilla, all of which appear to belong to the 

 same species — viz., Monstrilla dance, Claparede — have been obtained 

 during the past year. They are all from the Clyde district, chiefly Upper 

 Loch Fyne and Kilbrennan Sound. Usually one or two, rarely three or 

 four, specimens were obtained in a single gathering. 



The gatherings in which Monstrilla was observed were from the follow- 

 ing stations : — Station II. (three specimens), Station III. (one specimen), 

 Station IV. (two specimens), Station VI. (three specimens), Station 

 XIII. (two specimens), Station XIV. (four specimens), Station XVII., 

 two gatherings (one specimen each). These gatherings were all collected 

 in August and November 1898. 



