56 MEMOIR III. 



and to Nofticiila, and in structure coincides to such a degree 

 with the former, that many, of what are very improperly 

 termed rigid Linnreans, would he disposed to consider 

 them merely in the light of different species ; no Naturalist 

 however who understands any thing of the Crustacea, will 

 refuse his assent to their being generically distinct, (not 

 withstanding that we have only a male individual to con- 

 template) when the very peculiar structure of its sub-ab- 

 dominal fins is considered ; these in Mysis consist of a 

 siiigle joint, in Nocticula of three joints, while in Cynthia 

 they are intermediate, and composed of but tivo ; it is not in 

 the number oi ]oii\ts alone, however, that they differ, their 

 form and structure is also essentially different. In Cynthia, 

 the four last of these members are each composed of a 

 very large bilobate scale, supporting at its apex, two taper 

 articulate fins, strongly ciliated with plumose setae ; from 

 between these, originates an opaque organ, wliich bifurcates, 

 its two extremes of unequal length, being rolled inwards 

 the one over the other; the first pair differ in having 

 but one perfect terminal fin, with the rudiment of the 

 second and of the intermediate organ. 



In what part of the Atlantic the specimen of this animal 

 was procured has not been ascertained, but may be stated 

 in a general way, as somewhere in the usual track pursued 

 by West-Indiamen betweenMadeira and Barbadoes ; neither 

 is it known whether luminous or not, although presumed 

 to be of the former description, this circumstance there- 

 fore remains to be determined, as well as the difference of 

 structure in the female, and the mode of carrying her eggs. 



The only other difference which declares it to be gene- 

 rically distinct from Mysis, we find in the inner division of 

 the six posterior feet, which in place of the pluri-articulate 

 termination, have this part obscurely divided into two or 

 three joints only. 



