PORTUMNUS. 



83 



I have adopted some characters for this genus which 

 will imply the necessity of separating from it species which 

 have been included by Edwards in the genus Plati/onfcJius 

 of Latreille, which is synonymous with Portumnus of 

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

 handsome species, Platyonyclms hipustulatus, 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 Platyonyclms. 



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. 



Sterimm twice as long as it is 

 broad. 



Fifth pair of legs tvith the terminal 

 '■■ Joint broad oval, vary imicU rounded. 



Abdomen in the male five-jointed ; the 

 terminal joint not abruptly smaller than 

 the preceding one. 



Abdomen in tJie female seven-jointed ; 

 nearly three times as long as it is 

 broad ; the sides parallel, as far as the 

 fifth joint inclusive ; the tenninal 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 tvith the terminal ; 

 joint aeutely lanceolate. 



Abdomen in the male seven-jointed; 

 the tenninal 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. Li 



