20 MEMOIR II. 



have in this structure of the sub-abdominal fins, which is 

 not found in any other of the Macroura, another instance, 

 of the modifications which parts undergo in the hands of 

 Omnipotence, so as to adapt them to the peculiar habitudes 

 of the animal ; in Mysis they are wliolly adapted to swim- 

 ming, and therefore present the most simple arrangement, 

 but in the other Macroura, as they also serve to receive 

 and mature the ova, they are necessarily larger and more 

 complicate, and are each composed of two articulate 

 branches supported on a common basis. 



The Tail is composed of five scales, articulated to the 

 last segment of the abdomen ; the middle scale, varying 

 in shape and armature, has been considered by Dr. Leach, 

 as affording the best and most obvious specific characters ; 

 of the lateral scales, the outer ones are the largest, and 

 also present considerable difference in shape in the different 

 species. 



Of the different Species of Opossum Shrimp. 



Teie species of this Genus as yet known, are but few, 

 and appear to be principally littoral, frequenting the 

 shores and shallow parts of the sea, and the estuaries of 

 rivers. 



The Mysis Fahricii of Dr. Leach,* enumerated by Otho 

 Fabricius in his Fauna Groenlandica, under the title of 

 Cancer oculatus, and very imperfectly figured, (see Plate 

 II. fig. II, 12) is one of the longest known, and the type 

 of the Genus ; it has been lately more satisfactorily deve- 

 loped by Mons. Desmarets in his work " sur Ics Crustaces" 

 Plate 40, fig. 6. This species is distinguished by the middle 

 scale of the tail being obtusely and deeply notched, and at 

 the same time spinous on its edges : the outer scales of the 



* Mysis, Cauda lamella intermedia apice obtuse emarginata : lamellii 

 exterioribus apice rotundatis. Linn, Trans. Vcl. XI. p. 350. 



