tiQO DR. W. A. CUNNINGTON ON THE BRACHYUROUS [Mar. 5, 



sinuated. Post-frontal crest prominent, almost straight, and 

 extending to margins, with branches of mesogastric groove 

 angulated. Orbits large, and eyes lai-ge, with stout peduncles. 

 External orbital angle produced into prominent spine. Ischium 

 of external maxillipeds showing longitudinal furrow, somewhat 

 nearer to the inner edge. Chelipeds in both sexes subequal ; 

 merus trigonous, with a series of small' spines and distally a pro- 

 minent spine on the anterior margin ; carpus with two spines on 

 inner margin, the posterior being the smaller. Fingers pointed 

 and slightly hooked ; teeth fairly uniform, of moderate size. 

 Ambulatory legs long and slender, also somewhat compressed. 

 Colour in life dull dark green, shading into dark purple ; the most 

 prominent parts of the front, the external orbital spines, antero- 

 lateral margins, and portions of post-frontal crest outlined in 

 white ; legs dark purple. 

 Dimensions as follows : — 



Adult male (largest specimen) : 



mm. 



Length of carapace 38"4 



Breadth of carapace 56*9 



Fronto- orbital w idth 36"7 



Width of front 13-8 



Adult female : 



Length of carapace 36'9 



Breadth of carapace 53*9 



Fronto-orbitaJ width 34*5 



Width of front 14-2 



Remarks. — As there has recently been published a very com- 

 prehensive revision of the Potamonidse by Miss Rathbun*, it 

 becomes desirable to take this as a basis, and correlate with it any 

 newly described forms. Accordingly that portion of the key 

 given by Miss Rathbun't which is affected is restated below, but 

 slightly modified to include this new species, while distinguishing 

 it from jo/mstoni, the species close to which it finds a place. 



Ji'. Carapace broader ; less than three-quarters as long as broad. 

 J. Antero-lateral margins denticulated; external orbital angle 



produced into prominent spine orbitospinu/t. 



j' . Antero-lateral margins granulated ; external orbital angle 



forms low blunt process johnstoni. 



Even a casual study of the species comprised in the genus Poiftmoj* 

 renders it abundantly evident that in many cases they cannot be 

 distinguished by very striking difierences of character. It is 

 therefore the more important to make clear the actual points of 

 disagreement which have been thought in this" case to justify the 

 formation of a new species. P. orhitospinus approaches most 



* Nouv. Arch. Mus. Hist. Nat. Pans, 4' ser., vi.-viii., 1904-1906. 

 t Op. cit. vii. p. 160. 



