.Mr. C. Spencc Bate on the British Diastylida;. 453 



and, unlike the same segments in the higher forms, have the 

 dorsal ])ortion complete, and each is developed into a perfect 

 ring to which the respective thoracic legs are attached. 



The seven succeeding segments belong to the abdomen, and 

 are unfurnished with appendages, except the penultimate, which 

 is supplied with a pair of double-branched stylets, from which 

 peculiarity of form Say derived his generic name for the Ame- 

 rican species. h\ the male however the first two segments are 

 each supplied with a pair of short, stout, styliform appendages, 

 which are probably intremittent organs, since they homologize 

 with those which are known as such in the higher forms. The 

 last segment is developed into a caudal style, the edges of which 

 are furnished with short spinules : near the centre of this articu- 

 lation debouches the alimentary canal. 



The upper or interior antenna is short and pyriform, appa- 

 rently consisting of a peduncle, which is formed of a single seg- 

 ment and a short filamentary appendage of four articulations, 

 each being furnished with a long ciliated hair (PL XIII. fig. 4 a). 

 The whole organ does not extend beyond the rostrum-like pro- 

 jection of the carapace. 



The lower or exterior antenna is considerably longer than 

 the upper, and consists of a peduncle formed of a single joint 

 and a filamentary appendage, the first two articulations of which 

 probably homologize with the second and third joints of the 

 peduncle in the antenna of the true Macroura, since that which 

 we call the peduncle in this, evidently homologizes with the first, 

 or first and second, for the olfactory organ is distinctly dis- 

 cernible in the middle of the segment (PI. XIII. fig. 5 a), which 

 is considerably broader than the next succeeding ; we there- 

 fore think it convenient to describe it as the peduncle, and the 

 slighter continuation as the terminal filament, rather than accord- 

 ing to what may or may not be homologically true. The first 

 joint of the filament is nearly as long as the peduncle, and the 

 second considerably longer, whereas the four terminal are ex- 

 tremely short, each successively shorter than the preceding ; the 

 last is tipped with a slight brush of cilia, and reaches consider- 

 ably in advance of the most anterior extremity of the animal. 



The mandibles are the next succeeding pairs of appendages, 

 and are very powerful organs, not developed upon the ty]:)e of 

 the Decapoda, but furnished with a molar tubercle and a comb- 

 like row of teeth or hairs as found among the Amphipoda, pos- 

 sessing however a long osseous tendon as in the ]\lacroura, and 

 therefore forming a type intermediate between the two extremes, 

 and probably belonging to the Stomapoda (PI. XIII. fig. 6). 



The maxillae are thin foliaccous plates intermediate in their 



