BRACHYURA. 



417 



It differs from Neptunus (Achelous) granulatus in the following characters : (l) 

 The outer fissure of the supra-orhital margin is obsolete ; (2) the nine teeth of the 

 antero-lateral border gradually decrease in size from before backwards ; (3) the antero- 

 lateral angle of the merus of the external maxillipeds is rounded and but slightly 

 produced in a lateral direction ; (4) the chelipeds in the male are about twice the 

 length of the carapace, the posterior border of the arm is more expanded than in 

 N. granulatus, the anterior border of the 

 arm bears three well-developed spines and a 

 fourth inconspicuous one posterior to these, 

 the posterior border bears one spine only in 

 the position of the distal one of N. granulatus; 

 (5) the outline of the abdomen of the male is 

 triangular, abdominal terga II. and III. both 

 have well-marked carinae, across abdominal 

 tergum V. runs a transverse ridge just anterior 

 to the joint between terga V. and VI. this 

 is correlated with the form of the copulatory 

 appendages ; (6) the form of the male copu- 

 latory appendages is characteristic. 



It will be noted that in characters (1), (2) 

 and cheliped-length the new species agrees 

 with Neptunus (Achelous] orbicularis, but it 

 differs from the latter and resembles N. 

 granulatus in the granulation of its carapace 

 and chelipeds, in the open character of the 

 inner fissure of the supra-orbital margin, and in ratio C.b. -r-C.l. I have not seen any 

 specimen of N. orbicularis, but as points (3), (5), (6) and spines of hand, i.e., part of 

 (4), are not specified by ALCOCK as differences from N. granulatus in his description 

 of N. orbicularis (A. 4, p. 47), I conclude that the new species is sufficiently distinct 

 from the latter. 



The characters of the antero-lateral angle of the merus of the external maxilliped 

 may be conveniently used to separate N. dubia from the other two members of the 

 sub-genus given in ALCOCK'S key (A. 4, p. 32). 



I may note here that I judge Achelous rubro-marginatus, LANCHESTER (' Proc. 

 Zool. Soc./ 1900, p. 746, pi. xlvi., fig. 8), to belong to the sub-genus Amphitrite, linked 

 to the gladiator group by Neptunus (Amphitrite) petreus (A. 4, p. 37). The latter 

 species may have been unknown to LANCHESTER ; its description did not long precede 

 that of rubro-marginatus. 



Neptunus (Pontus) convexus, DE HA AN, 1833. 



Localities : South of Modragam, one specimen ; off Mutwal Island, two specimens ; 

 coral reefs, Gulf of Manaar, nine specimens ; pearl banks, eleven specimens. 



3 H 



Fig. 9. Neptunus dubia, n. sp., adult male. 



