DECAPODA. 199 



as in the second. It would also appear that tlie 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, Avhich 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 smallncss of the claws, 

 the large eyes, and lateral fins of the tail, wliich are composed of a 

 single lamina. Tliis tail is extended, narrow, and simply bent under 

 near its extremity. 



Megalopus, Leach. — Macropa, Lair., Encyc, 



Four species are known, three of which inhabit European seas, and 

 the fourth the Indian Ocean *, 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 termi- 

 nated by a didactyle hand. 



Their tail is always extended ; tlieir 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 fingers, 

 whose lateral antennre 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 conceal themselves 

 in holes which they excavate in the sand. 



Sometimes the index or immoveable finger, formed by a projection 

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

 the thumb or moveable finger, merely constituting a simple tooth. 

 The 



Gebia, Leach, 



Approaches the preceding sub-genera in the two anterior feet, 

 which are alone didactyle. The leaflets of the lateral fins of the end 

 of the tail widen from the base to their extremity, and are marked 

 with longitudinal ridges. The intermediate piece or the last segment 

 of the tail is nearly square f . 



Thalassina, Lat. 



The four anterior feet terminated by two fingers ; leaflets of the 

 lateral fins of the end of the tail narrow, elongated, and without 



* For the European species, see Desmar., Consid., p. 200 — •202, and pi. .xxxiv, 2 

 of the same work. 



f Thalassina litoralis, Risso, Crust., Ill, 2 ; — Gebia stelhita, Leach, Malac. Brit., 

 xx.xi, 1 — 9. See Desmar., Consid,, p. 203, 204. 



