GAMMARIDEA. 847 



It seems proper, that the three groups, just pointed out, although 

 united by transitions, should still be kept apart as subgenera of a 

 common genus, Orchestia ; and we therefore so retain them, naming 

 the subgenera Talitrus, Talorchestia, Orchestia. 



The Orchestiae have been described by previous writers as having 

 the superior antennas shorter than the base of the inferior pair. But 

 there are other species of similar habit, and alike in the caudal 

 stylets, mandibles, and maxillae, in which the superior pair of antennae 

 is longer than the base of the inferior. They are distinct from 

 Orehestise, moreover, in having the maxillipeds unguiculate, like the 

 Gammarids. These species, not before recognised as distinct, consti- 

 tute our genus Allorchestes. 



Structure. — The mandibles in the Orchestidae have a double denti- 

 culate summit, a lateral molar prominence, and a cluster or line of 

 setae on the space below the inner part of the summit. 



The inner maxillae have a number of stout serrulate or setulose 

 setae at the extremity, a slender inner branch terminating in two or 

 more longish setae, usually bent, and commonly a small, slender, one- 

 jointed outer branch, arising from near the middle of the outer side of 

 the main stem. 



The maxillae of the second pair consist of two oblong ligulate 

 lamellae, the outer properly a second joint to the other ; both have a 

 fine brush of shortish plumose hairs at summit, and on the inner they 

 extend partly down the inner side ; besides these hairs, there are two 

 longer setae on the inner side. 



The maxillipeds are lamellar, and terminate in an obtuse joint, fur- 

 nished with some short setae or hairs, in Orchestia, and with a stout 

 claw, and usually longer setae, in Allorchestes. 



The legs have the ordinary structure. The setae are in pairs as 

 usual along both margins ; those of the upper or outer margin of the 

 ten posterior pairs are usually shortest and sometimes obsolete. Along- 

 side of the pairs, there is often another smaller spinule, on one side or 

 both, sometimes a second; and rarely, there are scattered spinules 

 upon the surface between. The tarsi are slender, and have a minute 

 seta on the lower side towards apex, just below the base of the proper 

 claw, and often a minute projecting point on the upper side. They 

 are very similar in all the legs excepting in those of the fourth pair, 

 in which they have generally greater breadth and an angle or gibbous 

 prominence below. 



