GAMMARIDEA. 851 



In this view, the front of the head contains — 



e (epistome) and d, representing the sternal and episternal pieces 

 of the secmid-antennary annulus. 



I (labrura) and /, representing the sternal and episternal pieces of 

 the mandibular annulus ; and /' is the continuation on either side of 

 the same aunulus. 



The two annuli, the first and second maxillary, are not distinct; 

 but along the posterior side, we have parts of the maxilliped annulus. 



The sides and top of the head will hence correspond to the first 

 antennary and ophthalmic annuli, one or both. 



If the labrum (I) be considered an appendage to the epistome and 

 its annulus, the mandibular annulus would be unrepresented in the 

 front of the head. 



The structure in the Gammaridae is in general like that of the 

 Orchestiae. The most striking difference is the absence of the suture 

 separating the piece/, from the part above, e, has nearly the same 

 shape, though under considerable variations ; and in some cases it is 

 very prominently keeled, and the labrum also may be carinate, as in 

 Plate 62, fig. kf,g, h. The piece d is distinct, and it lies, as in Orches- 

 tia, just below the base of the inferior antennse, being plainly the 

 episternal piece of the second antennary annulus. The labrum (I) is 

 separated, as usual, by a deep suture from e. The mandibular palpus 

 in its retracted position extends upward over the epistome either 

 side of the medial line or carina. Compare also figures 5 h 7 c, Plate 64. 



The following are the characteristics of the genera and subgenera 

 of Orchestidae : — 



G-. 1. Orchestia. — Maxillipedes non unguiculati. Antennse lmae basi 2darum 

 breviores. Epimerae 5tae 4tis parce breviores. 



Subgen. 1. Talitrus. — Pedes lmi maris feminceve manu non instrueti. 

 Subgen. 2. Talorchestia. — Pedes lmi maris ac in Talitro, feminai manu 



parvula instrueti. 

 Subgen. 3. Orchestia. — Pedes lmi maris feminceque manu plus minusvc 

 instrueti. 

 G-. 2. Allorchestes, Dana* — Maxillipedes unguiculati. Antennse lmae minores, 

 basi inferiorum seepissime longiores. Epimerae 5tae 4tis saepius multo breviores. 



The very short posterior stylets readily serve to distinguish the 



* Amer. Jour. Sci. [2], viii. 136, 1849. 



