33 



The outer surface of the abdomen, like the pentagonal sternum, is granular. 

 Dimensions in mm.: 



Total length of carapace (including rostrum) 7.— 



Length of rostrum 0.6 



Breadth of carapace 7.9 



Distance between ext. orb. angles 5- — 



Length of chela 6. — 



Length of first pair of ambulatory legs 21. — 



Total length 

 Length of meropodite 

 Breadth of meropodite 

 Length of carpopodite ' 

 Length of propodite ') 

 Breadth of propodite 

 Length of dactylus 



of second pair of ambulatory legs 



27.- 

 9.25 

 i-75 

 3-75 

 6.— 

 1. — 

 6.— 



Length of posterior pair of legs S. — 



The incrustation of the animal is apparently very weak : the integument is thin, flexible 

 and transparent. 



The species of this genus have been caught at different localities of the Eastern Indian 

 Ocean. " Ptenoplax" notopus Alcock 3 ), the typical species, has been dredged near the Coromandel 

 coast and the Andamans, in depths of 180 — 45ometres; Doflein 3 ) records his species, R. chuni^ 

 caught at a depth of 614 metres, from the coast of West Sumatra, south of Nias; and finally 

 Mac Gilchrist + ) obtained his " Ptenoplax" dentata south-east of South Andaman Island, in a 

 depth of 502 metres. In comparing the descriptions and figures of Doflein and Mac Gilchrist 

 it seems to be more and more probable, that their species are really id ent ie al. 

 Indeed, the shape of the carapace, vvith the conspicuous supra-orbital tooth (which is called by 

 Doflein the external orbital angle) and the teeth at the lateral margins are exactly alike; both 

 authors agree in the description of the rostrum, of the furry coating of the carapace, and of 

 the much longer hairs at the front and antero-lateral borders. Again, there is no difference 

 between the figures with respect to the walking legs: the dactyli are relatively short, at least 

 shorter than the propodites, and the four longitudinal ridges, with which the dactyli, according 

 to Doflein, are provided, are also shown in Mac Gilchrist's figure. Dofleix's name being 

 published a few months earlier than Mac Gilchrist's should, then, have priority. 



The new species of the "Siboga" was dredged in a depth of 310 metres near the Kei 

 Islands, and so the genus Retroplitma (= Ptenoplax) may be included in the Indo-Malayan fauna. 



1) Measured along anterior margin. 



2) Alcock and Anderson, Journ. As. Soc. Bengal, v. 63, pit 2, 1894, p. 181, pi. 9, f. l—^a—b. 111. Zool. "Investigator", 

 Crust. pi. 15, 1895, f. 2 — 2<7 — b. Alcock, Account Deep-Sea Brachyura "Investigator", 1899, p. 79. 



3) Wiss. Erg. "Valdivia" Exp., Bd 6, Brachyura, 1904, p. 131, pi. 37, f. 1 — 2. 



4) Ann. Mag. Nat. Hist. (7) v. 15, 1905, p. 266. 111. Zool. "Investigator", Crust. prt II, 1905, pi. 74, f. 1. 



33 



SIBOGA- EXPEDITIE XXXIX C. 



