CANCROIDEA. 291 



Prselabial space or palate not divided on either side by a ridge. Inner 

 branch of first maxillipeds simple. Carapax narrow. Male abdomen 

 narrow at base. 



The following are the genera of this family with their character- 

 istics : — 



I. Carcinus, Leach. — Pedes postici male natatorii, tarso perangusto- 

 lanceolato. 



II. Portumnus, Leach. — Pedes 5ti natatorii tantum, tarso latiore, 

 lanceolato, acuto. Carapax non latior quam longus. 



III. Platyonychus, Latr. — Pedes 5ti natatorii tantum, tarso lato, 

 elliptico. Carapax latior quam longus. 



IV. Polybius, Leach. — Pedes 8 postici natatorii, tarsis late lanceo- 

 latis. 



The genus Xaiva of M'Leay (Smith's Illust. Zool. S. Africa, pi. 3), 

 is described as near Carcinus, the only distinguishing character men- 

 tioned being, that the third joint of the outer maxillipeds is subquad- 

 rate, carinate at base, and the inner margin is emarginated just above 

 its middle, where the next joint arises. This form of the joint occurs 

 in Platyonychus. The outer antenna are inserted, as in Carcinus, in 

 a fissure of the orbital margin, and the first joint is short and subtri- 

 angular. 



Platyonychus purpureus. 



P. bipustulato affinis. Carapax paulo transversus (latitudine quartd 

 parte major e); margine antero-laterali breviore, crass& 5-dentato; j route 

 4:-dentato, dentibus acutis, inter sese non wque remotis, medianis pro- 

 pioribus. Pedes anticifere cequi, carpo gramdis minutis reticulata et 

 scabricido, intus elongate" acuto, manu gramdis asperatd, inferne 

 transversim plicatd. Pedes postici margines densd ciliati, articulo 

 tertio superne" non denticidato, tarso fere duplo longiore quam lato. 

 Abdomen maris fere lineare, segmento penidtimo basin non latiore. 



Near P. bipustulatus. Carapax transverse (breadth one-fourth greater 

 than length) ; antero-lateral margin shorter than postero-lateral, 

 coarsely five-toothed ; front four-toothed, teeth acute, the two me- 



