igi7'] S- Kemp : Notes on Crustacea Decapoda. 245 



on the bases of the antennules and the buccal cavern is not limited 

 anteriorly by a ridge. The ischium of the external maxillipedes 

 is a little longer than the merus ; both segments are slender and, 

 when normally folded, gape in the middle line, the underlying 

 appendages being partially visible. In the abdomen of the male 

 the sutures of all the segments are distinct. The regions of the 

 carapace, as in most genera of the family, are defined by fine-cut 

 grooves. 



Numerous species have from time to time been placed in 

 Hymenosoma, but in the majority of instances the reference is 

 erroneous and it is now practically certain that the genus is mono- 

 typic. Stimpson's H. geometricum^ is synonymous with H. orbicu- 

 lare and Guerin Meneville's H. gandichaudii ,^ though included in 

 the genus by Milne-Edwards,* is evidently a species of Halicarcinus. 



Halicarcinus was established by White in 1846,* the type 



Fig. 2. — Halicarcinus plaiiatus (Fabr.). 

 Anterior part of carapace, seen from below. 



species being Fabricius' Leucosia planata^ from Tierra del Fuego. 

 In this genus the epistome"is a conspicuous plate, and the buccal 

 cavern is bounded anteriorly by a transverse ridge (text- fig. 2). 

 The ischium and merus of the external maxillipedes are of similar 

 size and are broad segments, completely or almost completel}' clos- 

 ing the buccal cavern. As in Hymenosoma the grooves on the 

 upper surface of the carapace are clean cut and, in the abdomen 



1 Stimpson, Proc. Acad. Sci. Philadelphia, X, p. 108 [54] (1858) and Smiths. 

 Misc. Coll., XLIX, p. 144 (1907). Stebbing-, in Marine Invest. S. Africa, IV, 

 p. 50 (1905) and Ann. S. African Miis., VI, p. 332 (1910), retains H. geometricum 

 as a distinct species, but has since agreed that it is synonymous with H. orbiculare 

 [see Trans. Roy. Soc. Edinburgh, L, ii, p. 270 (1914)]. 



^ Guerin Meneville, Voy.de la ' Coquille' , II, ii, l'"« div., p. 21 and Atlas. 

 Crust., pi. ii, figs. 12-18. 



'A Mihic-Edwards, Ann. Sci. nat., Zool., Paris (3), XX, p. 222 (1853). 



+ White, Ann. Mag. Nat. Hist, (i), XVIII, p. 178 (1846). 



6 For references see Stebbing, Proc. Zool. Soc. London, 1900, p. 524; Doflein 

 and Balss, Mitth. natiirhist. Mas. Hamburg, XXIX., p. ^5 (1912) and Chilton. 

 Subantarctic Is. of New Zealand, p. 609 (1910). 



