DECAPODA. 47 



Leptopus, Lam. 



Tail of the females composed of but five segments; the body con- 

 vex and feet very long. 



But a single species is knoAvn which is part of the collection of 

 the Museum d'Histoire Naturelle, where it is called Maia longipes. 

 Doctor Leach proposed to designate this genus by the name of Ste- 

 nopus, a denomination we have not adopted, inasmuch as it is al- 

 ready appropriated to another. That of Leptopus, Lam., is com- 

 posed of several species, which, the above mentioned one ex- 

 cepted, according to the characters here given, must be excluded 

 from it. 



If we except some species of Hymenosomse in which the tail pre- 

 sents but four, or at most five, distinct segments, that part of the 

 body consists of six in all the following sub-genera, either in both 

 sexes, or in the males. The third joint of the external foot-jaws is 

 sometimes in the form of an inverted triangle or of a posteriorly 

 narrowed oval, and sometimes in that of a heart. The ensuing joint 

 is inserted in the middle of its superior margin, or rather more out- 

 wards than inwards. 



Some of them, such as the three following sub-genera, approach 

 those of which we have just spoken by the almost isometrical, or at 

 least transversal form of the epistoma. The base of the interme- 

 diate antennae is but a short distance from the superior margin of 

 the buccal cavity. 



One of these sub-genera is distinguished from the others by the 

 flatness of the shell, and by the superior extremity of the first joint 

 (free in several) of the lateral antennae, which does not extend be- 

 yond that of the ocular pedicles. Such is the 



Hymenosoma, Leach. 



The shell is triangular or orbicular(l). The species are gene- 

 rally small and peculiar to the Indian Ocean and coast of Australia. 

 The number of caudal segments varies, but never extends beyond 

 six. 



In the two following sub-genera, the shell is more or less convex, 

 always triangular and terminated before in a rostrum. The first 

 joint of the lateral antennae, always fixed, forms a ridge or salient 

 line between the fossulse of the intermediate antennse and that of the 

 eyes, and which is prolonged beyond the end of the ocular pedicles. 

 In the 



(1) Hymenosoma orbicularis, Desmar., Consid., xxvi, 1 



