DECA PODA. rOR TUNIDM. 



BRACHYURA. 



GENUS POLYBIUS. Leach. 



PoLYBius. Leach, Edwards. 



Platyonichus. Latr. 



Generic character. — Extertial antennce with the basal joint 

 round, detached, moveable, with the remaining portion lodged 

 in a hiatus at the inner canthus of the orbit, which it does not 

 fill. Internal antennce in fossae, which are entirely open forwards. 

 'Eyes larger than their peduncles, which are short. External 

 pedipalps with the third joint subquadrate, longer than broad, 

 and slightly notched at its inner margin, near the anterior angle. 

 Carapace nearly orbicular, slightly contracted posteriorly. An- 

 terior pair of legs equal, the pincers curved. Secotid, third, and 

 fourth pairs compressed, the terminal joint flattened, thin, broad, 

 and lanceolate. The ffth pair with the penultimate joint much 

 flattened ; the terminal one very large, oval, foliaceous. Cara- 

 pace much depressed, the anterior margin semicircular. Orbits 

 with two fissures in the superior, and one in the inferior margin ; 

 a hiatus at the inner angle, and a small tooth at the outer. 

 Abdomen of the male, five-jointed, the first, second, and third 

 joints very short and broad, and transversely carinated ; of the 

 female, seven-jointed, the sides nearly parallel as far as the 

 middle of the sixth joint. 



The structure of this genus, of which a single species 

 only is known, is of a more decidedly natatory character 

 than any other brachyurous form found on our shores. It 

 is on this account that it lias been with great propriety con- 

 sidered as generically distinct from Portumnus^ with which, 

 however, it stands in very near relation. 



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