DECAPODA. 65 



The genus Monolepis, Say, — Journ. of the Acad, of Nat. Sc. of 

 Philad., I, 155; Desmar., Consid., p. 199 and 200, appears to con- 

 stitute the passage from the Porcellanas to the Megalopes. It ap- 

 proaches the first in the two posterior feet, and in the direction of 

 the tail. But this tail has but six segments, and the eyes are very 

 large as in the second. It would also appear that the lateral fins of 

 the end of the tail resemble those of the latter. 



The remaining Crustacea of the same division differ from the pre- 

 ceding in their posterior feet, which are similar to their preceding 

 ones in form, proportion and uses, or equally ambulatory. They 

 are also removed from them by the greater thickness and height of 

 the body, the shortness of the lateral antennae, the smallness of the 

 claws, the large eyes, and lateral fins of the tail which are composed 

 of a single lamina. This tail is extended, narrow, and simply bent 

 under near its extremity. 



Megalopus, Leach. — Macropa, Latr., Encyc. 



Four species are known, three of which inhabit European seas 

 and the fourth the Indian Ocean(l) whence it was sent to Paris by 

 the late M. Leschenault and Messrs Quoy and Gaymard. 



In our second division of the Astacini, Latr., will be comprised 

 those which have five pairs of false feet, the mediate antennae 

 straight or nearly so, salient, projecting, and terminated by two fila- 

 ments as long as their peduncle, or longer; and which, a single sub- 

 genus excepted — Gebia — have the four or six anterior feet terminated 

 by a didactyle hand. 



Their tail is always extended; their two posterior feet are never 

 more slender than the preceding ones, nor folded. The peduncle 

 of the lateral antennae is frequently accompanied by a scale. 



Some of them, as well as others of the ensuing section, inhabit 

 fresh water. 



Those in which the first four feet, at most, terminate in two fin- 

 gers; whose lateral antennae never have a scale at the base; and 

 where the external leaflet of the lateral fins of the end of the tail 

 presents no transverse suture, will form a first subdivision. Most 

 of their feet are ciliated or pilose. They inhabit salt-water and con- 

 ceal themselves in holes which they excavate in the sand. 



Sometimes the index or immovable finger — formed by a projec- 

 tion of the penultimate joint, of the claws, is very evidently shorter 



(1) For the European species, sec Desmar, Consid., p. 200 — 202, and pi. 

 xxxiv, 2, of the same work. 



Vol. III.— 1 



