10 MEMOIR I. 



it only remains to unfold this part of our subject, in 

 order to render their history complete, premising, that the 

 following description has been derived from one of the 

 full grown specimens above alluded to, which may there- 

 fore be supposed to differ from such as occur of smaller 

 size in the greater degree of developement of all its parts j 

 thus, the eyes are more distinctly pedunculate, the natatory 

 division of the feet have an increased number of plumose 

 setae, the rudiments of the sub- abdominal fins are quite 

 obvious, and the mandibles shew the rudiment of a palp ; 

 in other respects they are essentially the same. 



The Eyes are large, distant from each other, and 

 although on short footstalks, do not appear to be possessed 

 of any obvious motion. 



The Corselet, or cephalo-thoracic-clypeus covers the back 

 and sides of the animal, and is prolonged in front into a 

 long deflected spine, and has another long spinous projec- 

 tion on the posterior part of its dorsum, and a pair of 

 short lateral spines. 



The Abdominal portion uncovered by the clypeus, is 

 composed of four semicylindric narrow joints, each fur- 

 nished with the rudiment of a pair of sub-abdominal 

 fins, and is terminated by a deeply forked tail, spinous 

 within. 



The Antennas or feelers, are double on each side ; the 

 inner pair being short, and composed of two basil joints, 

 surmounted by two lobes, of which one lobe is very short, 

 and the other 3-articulate and setose ; the outer pair consist 

 of three parts, the central or principal one, large, long, 

 and taper, the lateral ones small and short, one of them of 

 3 articuli, and attached to the base of the central division, 

 the other also articulate, ending in two setae, and origi- 

 nating from the first joint of the principal. 



The Members or legs, consist of but tivo pair, each 

 divided into an outer and inner limb, of which, the outer 

 divisions are adapted to natation, and the inner to the 



