430 MR. J. K. HENDERSON— A CONTRIBUTION 



Genus Polyonyx, Stimpson. 



238. POLYONYX OBESULUS, Miei's. 



p. obesulus (White), Miers, ' Alert' Crust, p. 272, pi. xxix. fig. D (1884). 



Rameswaram and Tuticorin {Thurston). Common at Uameswaram, both free and in 

 sjwnges {J. B. II.). 



I have compared my specimens with the types in the British Museum. The median 

 frontal projection is obtusely rounded and but little prominent. The ambulatory dactyli 

 are triunguiculate, the middle claw being slightly stouter and larger than the distal 

 one, whereas the proximal one is much smaller. Sexual dimorphism is seen in regard to 

 the chelipedes and the width of the carapace. In both sexes the right chelipede (which 

 is usually the smaller of the two) has the hand more or less carinated inferiorly, and the 

 carina often minutely dentate ; the fingers are in contact throughout, or almost so, in 

 males, the opposing margins being finely dentate and without any prominent tooth. 

 In females and young males the left chelipede, which is as a rule slightly the larger, 

 is similar to the right, whereas in adult males it is more strongly developed ; the 

 fingers have a wide gape, and are not in contact even at the apices when closed; 

 while a prominent tooth is present on the inner margin of the lower finger. The male 

 probably holds the chelipede of the female during copulation. 



In P. hmngniculatus (Dana) the median frontal projection is prominent and acute, 

 while the ambulatory dactyli are biiinguiculate, the first or proximal claw, present in 

 P. ohesuhts, being scarcely visible and represented merely by a minute seta. On the 

 chelipedes the lobe of the inner margin of the merus is more prominent, and the carpus 

 is usually longer than in Miers's species. In some specimens the outer surface of the 

 hand is granulated. There are examples in the British Museum of Dana's species from 

 the Galf of Suez, the Seychelles, and the Amirantes. De Man has suggested that 

 P. ohesuhis is identical with P. hiungmcitlatus, and that the P. hiunguiculatus described 

 by Miers is a distinct species ; but I cannot agree with this suggestion. 



The following measurements are taken from Rameswaram specimens : — 



Male. 



Length of carapace 6'7 mm. 



Breadth „ 8'3 „ 



Length of left hand 11"5 „ 



Breadth „ 5-5 „ 



Length of left carpus 7"5 „ 



The colour is a pale red, turning white in spirit. One individual — a male — carries a 

 Sacculina. 



Distribution. Madjicosima Is. ( White) ; N. Australia [Miers, Henderson) ; Amboina 

 {Be Man); Singapore {Walker). 



