I D O T M I D E A. 703 



just anterior to it, so that the outline of the body either side is convex 

 throughout. Like the front, the centre of the caudal margin is apicu- 

 late. The outer antenna? have the first two joints shortest; the 

 second broadest, with the outer apex elongated ; the fifth joint little 

 longer than fourth. The whole length of the inner antennae scarcely 

 exceeds first three joints of outer. The claws of the legs have a 

 minute spine under the apex. The external plates of the abdomen 

 beneath are truncate behind. 



Figure 5 a, Plate 46, represents an animal found at the same time 

 with the preceding, and supposed to be a young individual of some 

 other species. It is broadest anteriorly, and has rather large eyes 

 situated on the lateral margin and containing about fifteen facets. 

 The head is transverse, and is not embraced behind by the following 

 segment; it is subtruncate in front, with the centre slightly project- 

 ing, and the sides rounded. Anterior thoracic segment very short, or 

 only partly visible. The thoracic segments increase in length from 

 the first to the last ; the three posterior are sublunate in an upper 

 view, the posterior margins being concave, and the angles prolonged 

 but obtuse, or nearly so. Abdomen five-jointed, the first four trans- 

 verse, and subequal in length ; the first abruptly narrower than the 

 following or preceding thoracic segment; the fifth oblong, shield- 

 shape, the sides curving and meeting behind in an angle. The exte- 

 rior plates of the abdomen have a triangular subobtuse termination. 

 Inner antennae less than half the length of the outer ; the third joint 

 smaller than the second. The outer antennae consist of seven joints, 

 the first five of which correspond to the basal portion ; the first three 

 joints are quite short ; the second has the outer apex prolonged the 

 length of the third joint ; the fourth and fifth joints are a little 

 oblong, but shorter than the sixth and seventh ; the last is pointed. 

 A few short hairs on the joints. Legs subequal ; increase from the 

 first pair to the last. The claw nearly straight, has a largish sub- 

 conical base. 5 b, represents the leg of the fifth pair ; 5 c, the plate 

 covering the branchial leaflets. 



Length, one line. Colour, brownish gray. 



Several specimens were taken December 23, 1838, in the harbour 

 of Rio, near the city. 



