38 



Group III. : pealianum group. 



In this group- -which includes P. pealianum, P. antennarium, P. twnidmn, 

 P. turgidulum, and perhaps P. tumidulum — the cervical groove takes the same 

 course, and cuts the post-orbital crest at the same point, as in Groups I. and 

 II. ; biit the carapace is deep (its depth lieing half its length) and has a marked 

 convexity fore and aft. 



The species may be tabulated as follows : — 



I. Front with a bilobed eminence immediately behind and parallel with its 

 edge ; size large : — 



1. Antennal peduncle in contact with edge of front as usual . . P. pealianum. 



ii. Antennal peduncle quite clear of front . . . .P. aniennarittm. 



II. No circumscribed bilobed eminence at edge of front ; size small, the adult 

 carapace about an inch long : — 



i. Flagellum of exopodite of external maxillipeds of normal size : — 



1 . Post-orbital crest blunt ; 2nd pair of legs not longer than larger 



cheliped . . . . . . . .P. tumiduiit. 



2. Post-orbital crest sharp ; 2nd pair of legs longer than larger 



cheliped . . . . . . . .P. turgidulum. 



ii. Flagellum of exopodite of external maxillipeds extremely short ; 



post-orbital crest thick, blunt, and wrinkled . . .P. iiimidiilmn. 



P. tumidulum should perhaps be placed in the section, or subgenus 

 Potamiscus, as the exopodite of the external maxillipeds has only a vestige of a 

 flagellum. 



18. PoTAMON (Potamon) PEALIANUM, Wood-Mason. (Fig. 44.) 



Telphma pealiana, Wood-Mason, Journal Asiatic Soc, Bengal, XL., 1871, pt. II., p. 204, pi. xiv., figs. 7-11. 

 Potamon (Potamon) pealianm, M. J. Rathbun, Nouv. Archiv. du Museum, Paris (4), VI., 1904, p. 310 {nl,i lit.). 



This species differs from all the foregoing in the following particulars : — 



(1) The carapace is deeper, its depth being half its length, and is very 

 much more convex fore and aft ; 



(2) The edge of the front is vertically deflexed, and in a dorsal view is quite 

 concealed by a bilobed frontal eminence ; 



(3) The propodites and dactyli of the legs are longer. 



From the Jiuviatile group it further differs — (4) in the much more distinctly 

 areolated carapace ; and (5) in the form and proportions of the merus of the 

 external maxillipeds, which is as long as broad and roughly hexagonal. 



From the atkinsonianum group it further differs — (4) in the form and pro- 

 portions of the merus of the external maxillipeds, as above ; (5) in the more 

 superficial grooving and less prominent sculpture of the carapace, the edge also 

 of the front and orbits being smooth, or nearly smooth, instead of crenulate ; 

 and (6) in the more salient lateral epibranchial tooth. 



