1899.] A. Alcock — Carcinological Fauna of India. 47 



27. Neptunus {Achelous) orbicularis, Richters. 



Achelous orbicularis, Richters in Mobias Meeresf. Maurit. p. 153, pi. xvi. figs. 

 14, 15: J. R. Henderson, Trans. Linn. Soc, Zool., (2) V. 1893, p. 371. 



Differs from Neptunus (Achelous) granulatus ia the following cha- 

 racters : — 



(1) the carapace is extremely thia and depressed, is perfectly- 

 smooth — except for faint marginal depressions — and bare, and is sub- 

 circular in shape, its length being f its breadth : 



(2) the outer fissure of the supra-orbital margin is obsolete, and 

 the inner fissure is represented by a closed suture : 



(3) the antero-lateral borders are cut into 9 teeth which gradually 

 decrease in size from before backwards : 



(4) the chelipeds in the male are about twice the length of the 

 carapace, their surface is non -granular, and the posterior border of the 

 arm is more expanded than in N. granulatus. 



In the Indian Museum are 6 specimens from the Pedro Shoal 

 (Laccadives) and 2, including an egg-laden female, from the Andamans. 



Chartbdis, De Haan (Goniosoma, A. Milne Edwards). 



Charyhdis and Oceanus, De Haan, Fann. Japon. Crust, pp. 10, 9. 

 Goniosoma, A. Milne Edwards, Ann. Sci. Nat., Zool., (4i) XIV. 1860, p. 263, and 

 Archiv. da Mus. X. 1861, p. 367 : Miers, Challenger Brachynra, p. 189. 



Charyhdis, M. J. Rathbun, Proc. Biol. Soc. Washington, XL 1897, p. 161. 



Carapace hexagonal, moderately broad, depressed or little convex, 

 usually with transverse granular ridges at any rate in its anterior 

 half. 



Front proper (not including the inner supra-orbital angles from 

 which it is distinctly separated) usually between a fourth and a third 

 the greatest breadth of the carapace, cut into six lobes or teeth (exclu- 

 sive of the supra-orbital angles). 



Antero-lateral borders oblique, moderately arched, longer than the 

 postero-lateral, cut into from 5 to 7 — usually six — teeth including the 

 outer orbital angles. 



Upper border of orbit with two notches or fissures ; there is a gap 

 in the lower border, and the inner angle of this border is usually 

 dentiform and moderately prominent. The antennules fold trans- 

 versely. 



Basal antenna-joint short and broad ; its outer angle forms a 

 lobule which usually fills the orbital hiatus and meets the front, exclud- 

 ing the flagellum from the hiatus. 



Epistome sufficiently long : buccal cavern squarish, broader than 

 long : the efferent branchial canals usually well defined. 



