76 MEMOIR IV. 



others previously seen by a solitary observer (Argulus 

 armiger)and almost lost sight of byNaturalists and excluded 

 from their works ; others actually inhabitants of fresh water 

 and quite accidental (Polyphemus oculus) ; some not 

 commonly observed (Megalopee) ; others perfectly nondes- 

 cript and incapable of being associated in any of our 

 classifications of the Crustacea ; of this description is the 

 little animal about to be described : besides these were 

 many others of inferior note, as Cyclops, Praniza, exuvia 

 of Tritones, &c. 



Without dwelling upon the advantages of such an 

 unexpected and valuable accession to a person in pursuit 

 of the Natural History of Marine animals, the above 

 nondescript and anomalous genus, as it was thought at the 

 time, shall be first described, and then the facts which the 

 last season has furnished towards the completion of its 

 history. 



This is a small translucent animal one tenth of an inch 

 long, of a somewhat eliptic form, but very slightly compres- 

 sed laterally, and of a brownish tint. When in a state of 

 perfect repose jt resembles a very minute muscle and lies 

 upon one of its sides at the bottom of the vessel of sea 

 water in which it is placed ; at this time all the members 

 of the animal are withdrawn within the shell, which appears 

 to be composed of two valves united by a hinge along the 

 upper part of the back and capable of opening from one 

 end to the other along the front, to give occasional exit to 

 the limbs. The limbs are of two descriptions, viz. anteriorly 

 a large and very strong pair, provided with a cup-like 

 sucker and hooks, serving solely to attach the animal to 

 rocks, stones, &c- and posteriorly six pair of natatory mem- 

 bers, so articulated as to act in concert and to give a very 

 forcible stroke to the water, so as to cause the animal when 

 swimming, to advance by a succession of bounds, after the 

 same manner as the water-flea (Daphnia) and other Mono- 

 culi, but particularly Cyclops whose swimming feet, are 



