47 



PoTAMON (Potamon) larnaudii, A. Milne Edwards. 



Telplmsa larnaudii, A. Milne Edwards, Nouv. Archiv. du Museum, Paris, V., 1869, p. 166, pi. x., fig. 4. 

 Potamon (Potamon) larnaudii. M. J. Rathbun, Nouv. Archiv. du Museum, Paris (4), VI., 1904, p. 275, pi. x., 

 fig. 7 ()//'(■ lit.). 



This species differs from all the preceding in the fact that the cervical 

 groove does not cut the post-orbital crests. The post-orbital crests are, indeed, 

 usually ragged or broken into tubercles near their outer end, but not by the 

 cervical groove. The cervical groove which is deep posteriorly, becomes broad, 

 superficial, and then faint anteriorly, and so far as it can be traced it runs to 

 the lateral epibranchial tooth, without branching to cut off a lobe of the 

 post-orbital crest. Another character, upon which Miss Eathbun lays stress, is 

 the breadth of the posterior lobe of the mesogastric areola, which is a third (in 

 large individuals not quite a third) the breadth of the carapace. 



Carapace slightly convex fore and aft, its length a little over three-fourths its 

 greatest breadth, its depth about three-sevenths its length. The gastric region 

 is not particularly well defined ffom the epibranchial regions anteriorly ; the 

 anterior part of the mesogastric region is very narrow. There is no very 

 distinct areolation of the epibranchial regions. The smface of the carapace is 

 profusely pitted ; the anterior part of the frontal region is granulous, the 

 epigastric crests are rugulose, and there are some tubercles and oblique 

 wrinkles on the antero- lateral part of the epibranchial regions, but otherwise 

 the dorsum is very smooth ; the side-walls of the carapace are marked with fine 

 oblique rugce, which also invade the postero-lateral borders ; the suborbital 

 lobes are smooth or sparsely granulous ; the pterygostomian regions are smooth 

 or at most granulous along the outer edge. 



The ft"ont in adults is less than a third the greatest breadth of the carapace ; 

 its edge is smooth, well defined, and broadly bilobed, the lobes being sometimes 

 a little sinuous. The outer orbital tooth is well defined and is separated fi'om 

 the lower border of the orbit by a gap. 



Antero- lateral borders of the carapace about as long as the postero-lateral, 

 well defined, gently curved, rather irregularly crenulate. Lateral epibranchial 

 tooth distinct. 



The epigastric crests are oblique, but their outer end is in the same line 

 with the post-orbital crests ; they are separated from the latter by a groove. 

 The post-orbital crests are nearly transverse, almost trenchant except at their 

 outer end, where they are rugose or actually broken up into tubercles ; the 

 cervical groove does not cut them. 



In the male abdomen the length of the 6th segment is between half and two 

 thirds its greatest breadth, and the 7th segment is not quite as long as broad. 



In the external maxillipeds the merus is as long as, or nearly as long as, 

 broad, otherwise the mouth-parts are as in P. fliiviatile. 



