The following key is proposed to distinguish the genera : 

 i. Epistome absent; ext. maxillipeds almost in contact with the 

 bases of the antennules. Rostrum prominent, pointed, 

 concave transversely. Dactyli of walking legs very long, 

 styliform, not depressed, almost wholly straight and ending 

 in an acute point, with two longitudinal rows of hairs and 



without spines at inner side Hymenosoma Desmarest 



Epistome present. Dactyli of walking legs with spines at 

 inner side . 2 



2. Epistome rather long in longitudinal direction. Antennulae 



and antennae almost completely concealed beneath the 

 rostrum; the latter projecting (as also the lateral borders 

 of the carapace), triangular, truncate or tridentate. The 

 ext. maxillipeds completely close the buccal cavern ; merus 



shorter than ischium 3 



Epistome short in longitudinal direction. Antennulae and 

 antennae only at base concealed by the rostrum, that is 

 triangular, trilobate or trispinose, but marked off poster-^ 

 iorly by a raised rim, that borders the flattened part of 

 the carapace and is continued across the base of the 

 rostrum. Ext. maxillipeds slightly gaping; merus somewhat 

 longer than ischium. Sides of carapace with one or two 

 (very rarely more) teeth at either side, but sometimes 

 unarmed 4 



3. Rostrum elongate, triangular. Epistome very long, somewhat 



convex. Antennulae separated by a mere ridge . . . Trigonoplax H. Milne-Edwards 

 Rostrum short, triangular, trilobate or truncate. Epistome 

 shorter. Antennulae separated by a more or less prominent 

 septum Elamena H. Milne-Edwards 



4. Rostrum consisting of three equidistant and equal, obtuse 



lobes, between the intervals of which the antennulae are 

 visible. Sides of carapace with two teeth on either side. 

 Walking legs rather robust, not much longer than breadth 



of carapace Halicarcinus White 



Rostrum trilobate (the median lobe being inserted beneath 

 the level of the lateral ones), triangular, truncate or tri- 

 spinose. Sides of carapace unarmed or toothed. Walking 

 legs mostly very slender, spider-like, elongate, but some- 

 times short and thickly hairy 5 



5. Walking legs very thin and slender, much elongate; dactyli 



falciform, of nearly equal breadth till near tip. Rostrum 



4 



