Crustacea. 5287 



This is the smallest and yet stoutest of all the species of Mysis with 

 which I am acquainted. It is generally very easy of detection, by its 

 being most commonly of a deep arterial blood -colour, especially 

 towards the posterior portion of the carapace and in spots along the 

 abdomen. All the specimens yet examined have been thus more or 

 less marked ; but yet colour alone can never, among Crustacea, be 

 taken as confirmations of specific differences. It rarely exceeds 

 three-fourths of an inch in length, though occasionally a few may be 

 found rather more than this. It is a very light and active species, 

 but much more quiet in its movements than the others. The cara- 

 pace is wider in proportion to the abdomen than in M. vulgaris or M. 

 Chamelion, and it is more enlarged posteriorly than in any of the other 

 long-tailed Crustacea. The rostrum is prolonged into an obtuse, tri- 

 angular snout, which is about one-third as long as the peduncle of the 

 eye. The internal antennae terminate in two multiarticulate fila- 

 ments; the basal joints visible are three in number; the lowest is 

 smallest ; the second about twice as long as it is wide, and reaches 

 as far as the peduncle of the eye; the third or terminal one is much 

 compressed superiorly, and expanded for the articulation of the ter- 

 minal filaments. The internal antennae have two terminal filaments, 

 and are long and large, and the upper part of the basal margin of the 

 large filament is covered with reflected hairs ; the second is shorter 

 and stiff. The external antennae are longer than the animal, and ge- 

 nerally stand almost at right angles from the body. The antennal 

 scale is long and obtusely triangular, the apex being rounded ; the 

 internal and external edges are both clothed with long, closely 

 arranged hairs, and there is no spine on the anterior termination of 

 the external margin. The abdomen is slightly longer than the cara- 

 pace, and in some specimens about one-third longer. The central 

 caudal plate is about half as long as the second and deeply bifur- 

 cated ; its external margins are nearly straight, the internal slightly 

 waved, and the whole marked with deep serrations ; but at the exter- 

 nal angle are two stouter teeth than the rest, diverging from one ano- 

 ther. The two external plates are long, slender, rounded at their free 

 extremities, and surrounded by long, closely arranged hairs. 



The only British species with which this can be confounded is 

 Mysis Chamelion of Bell ; but there are, I think, sufficient marks of 

 distinction in each to justify a separation into two species. In the 

 one now under notice the antennal scale is not more than half as long 

 as in M. Chamelion, and the distal margin is plain and obtusely trian- 

 gular; while in M. Chamelion it is truncated, and armed with a long 



