234 



Memoirs oj tlie Indian Museum. 



[Vol. V, 



IS 



Genus PTYCHOGNATHUS, Stimpson. 

 Ptychognathus onyx, Alcock. 



1900. Ptychognathus onyx, Alcock. Journ. Asiat. Soc. Bengal, LXIX, p. 404, and Illnst. Zool. 



'Investigator ', Crust., pi. Ixv, figs. 2, 2a (1902). 

 1905. Ptychognathus onyx, de Man, Proc Zool. Soc, London, II, pp. 542-544 (key to species).' 



This species, hitherto known from two j^oung males "probably from Tavoy 

 represented in the Chilka Lake collection by two females and three adult males. 



Due allowance being made for age and sex, the examples agree closely with the 

 type specimens and with Alcock' s description and figures. 



In the adult male, as in the younger type specimens, the exopod of the outer 

 maxillipedes is scarcely broader than the endopod. In the chelipedes there is a dense 

 patch of hair on the lower surface of the merus at its inner and distal ends^, the carpal 



spine is very strongly developed and the 

 chelae, which are greatly swollen, are nearly 

 as long as the carapace. The palm is covered 

 externally with microscopic granules, so ar- 

 ranged as to form a reticulate pattern and 

 the surface is rather conspicuously puckered 

 near the point of attachment of the carpus. 

 The fingers are two and a half times the 

 length of the upper border of the palm; 

 though pointed, they are slightly hollowed 

 at the tip and are provided with a series of strong teeth, one of which, near the 

 middle of the fixed finger, is larger than the rest. On the outer surface of the 

 palm a conspicuous and slightly sinuous ridge extends from the base to the tip of the 

 fixed finger and the space between this ridge and the teeth in the finger cleft is 

 occupied by a dense patch of hairs similar to that on the merus (text-fig. 11). The 

 mobile finger is not grooved and does not bear a patch of hair, thus differing from 

 P. barbains (A. Milne-Edwards) and P. piisillus, Heller. 



In the abdomen of the male the penultimate segment is very much broader than 

 the ultimate and its distal angles are sharply and obliquely truncate (text-fig. 12). 



Males of this genus are readily determined by the use of the excellent key which 

 de Man has supplied (op. cit.). In that which he has given for the identification of 

 females, the present species is omitted, the sex being up till now unknown. Females 

 of P. onyx would in this key take place alongside P. dentata, de Man, both being 

 separated from P. riedeli, A. M.-Edw., and P. andamanicus , Alcock ■, by the much 



Fig. 11— Ptychognathus onyx, Alcock. 

 Chela of male, external view. 



' Since this key was published two other species have been described: P. easterana, Rathbun, 

 Mem. Mm. Comp. Zool., XXXV, p. 31, pi. ii, fig. 4 ; pi. vii, figs. 4, 4a (1907) and P. johannae, Rathbun, 

 Proc. U. S. Nat. Mus., XLVI, p. 354, pi. xx.x. The latter species, said to be closely related to P. 

 riedeli, is apparently still more closely allied to P. barbains and P. pusillus. 



^ This patch is also to be seen in the type specimens, but is not nearly so well developed. 



^ De Man has suggested that P. andamanicus and P. riedeli are synonymous. I have examined the 

 types of the former species, but have not seen examples of the latter. 



