237 



These differences may easily be accounted for by the various stages of age, though it 

 must be admitted, that a serious objection to this assumption is lying in the differences between 

 the meri of the external maxilHpeds in the two forms. Supposing for the present, that "B" is 

 the yoLinger stage of "A", we arrive at the conclusion that, though in all stages of age 

 allowance is made for individual variation with regard to the shape of the front, the growing 

 individual, which appears to show a preference to a shallower depth than his grown-up 

 parents ^), goes through some modifications, both in the shape of the external maxillipeds and 

 in the armature of the fingers of the left chela. This young stage or "forma B" is also generally 

 distinguished by a ruddy-brown colour, covering to a greater or lesser extent the under and 

 side parts of the body and the walking legs. 



At one station (Stat. 312) 2 cf were caught, at a depth of 274 metres, one belonging 

 to "A", the other, much smaller one, to "B". For the rest, at all stations either "A" or "B" 

 was obtained, i. e. "A" in much deeper water than the other form. At one station (Stat. 302) 

 a very small specimen (breadth across carapace only 3.5 mm.) lived, at a depth of 216 metres, 

 which presents a mixture of characters of both forms: it has the general colour and the left 

 chela entirely like in "B" or in the young stage, but the external maxillipeds are shaped like 

 those of "A". Leaving aside this specimen, the "forma A" was found at Stat. 5, 254, 306 

 and 312, the other at Stat. 114, 116, 260 and 312. 



The species has been recorded from the Andaman Sea, the west coast of Sumatra and 

 the Gulf of Siam. 



Dimensions in mm. : i ' I ^ 



Breadth of carapace 11.5 ! 8.15 



Length of carapace 10.5 6.6 



Length of left chela | — 1 11. — 



Length of second pair of walking legs . I — 13. — 



4-5 

 5-4 



4 



3-5 

 3-1 



N" I is the egg-bearing specimen of Stat. 5 (diameter of eggs 0.73 mm.), n" 2 from 

 Stat. 254, n" 3 ("forma B") from Stat. 312, n" 4 (intermediate between forma "A" and forma 

 "B") from Stat. 302. 



Subfam. Hexapodinae. 



It would seem at first sight that no subfamily among all the Brachyura could be more 

 sharply characterized than the present one, on account of the entire lack of the fourth pair of 

 walking legs. And yet there is some controversy among systematists with regard to the limits 

 of the group, for species with truly three pairs of walking legs are, by means of Amorphopus^ 

 in which the fourth pair is represented by a minute tubercle on the coxopodites of the third 

 pair, connected to such genera like Psezcdopinnixa, Tritodynamia etc, with the fourth pair, 

 though small and weak, distinctly developed. This fact induced Ortmann -), who considered 



1) Miss Rathbun records a single specimen (^(^) from the Gulf of Siam, taken at a depth of only 20 fathoms, but she does 

 not give any description of this specimen. Alcock's three specimens are from deep water (194 fathoms) and all probably adult. 



2) Zool. Jahrb., Syst., Bd 7, 1894, p. 690 — 691. 



89 



SIBOGA-EXPEDITIE XXXIX c'. 31 



