MEGALOPIDEA. 435 



and fills a deep channel in the sternum, as in the Brachyura, it has a 

 pair of caudal appendages, like the Anomoura of a lower grade, though 

 with but a single plate to each appendage. They are, therefore, supe- 

 rior to the Anomoura Media in the cephalic portion of the body, and 

 somewhat inferior to them in the abdomen. Besides, they have not 

 the longitudinal suture near the sides of the carapax common to all 

 Brachyura, and in this respect they are Macroural. 



The Megalopidea embrace the two published genera, Megalopa and 

 Monolepw; and to these we add three others, Marestia, Cyllene, and 

 Tribola. In all, the carapax is rostrate, and except in Cyllene and 

 usually also Megalopa, it has a prominent point either side of the 

 beak, so that the front is tricuspidate ; the inner antennas are longi- 

 tudinal or oblique, and fold up into fossettes between the beak and 

 the cusp or tooth either side ; the beak is canaliculate above. In 

 Monolepis and Marestia, the beak is deflexed, so as not to be visible in 

 an upper view. In Cyllene and Megalopa, the beak projects nearly 

 horizontally, or is obliquely deflected. In Tribola, the beak projects 

 nearly horizontally, and also the cusp either side, and the inner an- 

 tennas are seen in an upper view, very nearly as in the Plagusiae. 

 Thus, although the general form of the carapax in these genera is 

 different, still all belong to one type. The outer antennas are also 

 similar. In the species of the different genera examined, they have 

 a three-jointed cylindrical base, and an eight-jointed flagellum, with 

 commonly two or three long setae at the apex of the fifth of the joints 

 of the flagellum. The joints are then, first, three basal; thence of 

 the flagellum, with the long setae at the apex of the last ; then three 

 terminal. The numbers five and three are typical, as in other Crus- 

 tacea. 



We have not had an opportunity of studying either those species of 

 Megalopa of Leach in which the beak is nearly horizontal, or the M. 

 mutica, in which it is deflected vertically.* In the genera Monolepis, 

 Marestia, and Cyllene, and probably also in Tribola, the posterior legs 

 are capable of overlying the posterior angle of the carapax, and there 

 is a corresponding depression in this surface, which depression is some- 

 times abrupt and channel-like. The penult pair of legs also admits 

 of being thrown forward over the border of the carapax, and extends 



* Leach describes three other species (not noticed by Edwards) in Tuckey's Expedition 

 to the Zaire (London, 1818), p. 404. The M. Cranchii may be a true Megalopa; the 

 others have a deflexed beak. 



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