PORTUMNUS. 



83 



I have adopted some characters for this genus which 

 Avill imply the necessity of separating from it species which 

 have been included by Edwards in the genus Platyonychus 

 of Latreille, which is synonymous with Portumnus of 

 Leach. The general form and habit of a large and very 

 handsome species, Platyonychus bipustulatus, Edw., must 

 at once strike even a casual observer as very distinct from 

 our species, on which Dr. Leach founded his genus ; and 

 the details of many important organs will offer no less 

 striking discrepancies. I will now venture to place before 

 the reader some of these points in a parallel view, pre- 

 mising that I propose to consider our species as the type 

 of the genus Portumnus, and the other as that of a distinct 

 genus, for which I would retain Dr. Milne Edwards's name 

 of Platyonyclius. 



PORTUMNUS. 



Carapace quite as long as it is broad, 

 with the latero-anterior margins very 

 slightly toothed ; the front tridentate. 

 Orbits with a single fissure in the 

 upper margin. 



Sternum twice as long as it is 

 broad. 



Fifth pair of legs with the terminal 

 joint broad oval, very much rounded. 



Abdomen in the male five-jointed ; the 

 terminal joint not abruptly smaller than 

 the preceding one. 



Abdomen in the female seven-jointed ; 

 nearly three times as long as it is 

 broad ; the sides parallel, as far as the 

 fifth joint inclusive ; the terminal joint 

 not abruptly smaller than the preced- 

 ing one. 



PLATYONYCHUS. 



Carapace one fourth broader than it 

 is long ; the latero-anterior margin very 

 strongly toothed ; front quadridentate. 

 Orbits with two fissures in the upper 

 margin. 



Sternum not more than one third 

 longer than it is broad. 



Fifth pair of legs with the terminal 

 joint acutely lanceolate. 



Abdomen in the male seven-jointed; 

 the terminal joint abruptly smaller than 

 the preceding one. 



Abdomen in the female seven-jointed; 

 not half as long again as it is broad ; 

 the fourth, fifth, and sixth joints form- 

 ing nearly a circle, posteriorly trun- 

 cated ; the terminal joint only one- third 

 the breadth of the preceding one. 



Such are some of the most important characters in which 

 these two forms differ, and on which I have thought it 

 necessary to consider them as generically distinct. In 



G a 



