270 REV. T. R. R. STEBBING ON STALK-EYED CRUSTACEA MALACOSTRACA 



distinction." But at least in the typical species of each genus the third maxillipeds 

 are very unlike, the third and fourth joints being about twice as long as broad in 

 Hymenosoma orbicularis, whereas the length and breadth are subequal in Hali- 

 carcinus planatus ; besides that the details of shape are not the same. A further 

 dissimilarity concerns the ringers of the walking-legs, which in H. orbicularis have 

 the inner margin of the fingers smooth, but clothed almost to the tip with two dense 

 rows of long plumose setae inserted on either side. In H. planatus these ringers 

 have the inner margin fringed with spine-teeth alternating with setse of very 

 moderate length. In the figures and description which Mr W. H. Baker supplies in 



1906 for Hymenosoma rostratus, Haswell, he represents a maxilliped agreeing with 

 that of H. planatus, and says that the dactyli of the ambulatory legs " are slightly 

 curved and carry a series of small teeth of about equal size with hairs between." Thus 

 the species seems unsuitable for union with Hymenosoma. Professor Chilton in 



1907 decides that Hymenosoma depressus, Jacquinot, belongs to the genus in which 

 Jacquinot placed it, and though Jacqtjinot's figure of the external maxilliped is 

 obviously untrustworthy, the limbs in Chilton's specimen had the other mark of the 

 genus in " the fact that the terminal joints of the last four pairs of legs were fringed 

 with hairs and looked as if they were used as swimming organs." 



Hymenosoma orbicularis, Desmarest. 



Plate XXVa. 



1825. Hymenosoma orbiculare, Desmarest, Consid. gen. Crust., p. 163, pi. xxvi. figs. 1, la-e. 



1837. „ „ Milne-Edwards, Hist. Nat. Crust, vol. ii. p. 36. 



1838. (?) „ ,, Milne-Edwards, Regne anim., illustr., pi. xxxv. figs. 1, la. 

 1858. Hymenosoma geometricum, Stimpson, Pr. Ac. Philad., p. 108 (54). 



1904. Hymenosoma orbiculare, Doflein, Deutsch. Tiefsee-Exp., vol. vi. p. 88. 



1905. Hymenosoma geometricum, Stebbing, Gilchrist's Mar. Invest., "S.A. Crust.," part iii. p. 50. 

 1907. Hymenosoma geometricum and Hymenosoma orb ictdare, Stimpson, Smithson. Misc. Coll., vol. xlix. 



p. 144. 

 1910. Hymenosoma orbiculare and Hymenosoma geometricum, Stebbing, Ann. S. Afr. Mus., vol. vi. part iv. 



pp. 331, 332. 

 1913. Hymenosoma orbiculare, var. geometricum, Balss, Schultze, Forschungsreise in Siid Afrika, vol. v. 



part ii. p. 113 (64). 



It may seem presumptuous to cancel Stimpson's species, but it was founded on a 

 single small specimen, and his remark that the " feet are sparsely provided with fine, 

 inconspicuous hairs," may have been due to a quite accidental condition. Desmarest 

 gives no indication of the fringe of short setae with which the broad end of the 

 female pleon is beset. His figure of the external maxillipeds shows the third joint 

 much smaller than the fourth, but this misleading error is corrected by Milne- 

 Edwards in the illustrated edition of the Regne animal. In the chelipeds of this 

 species the hand is much more swollen in the male than in the female, and the 

 movable finger of the male has a tooth on the inner margin which appears to vary 



