376 CEYLON PEARL OYSTER REPORT. 



Variability of rostrum-length is high. Thus for eight ovigerous females the index 

 K.I. -r G.I. has mean value = 0'47, and range of variation from 0*32 to 0'59. For the 

 ten males of various ages the corresponding figures are 0'57 and 0"37-0*77. 



Specimen *' (female) stands apart from the others and makes some approach to 

 Huenia proteus in the character of its last pair of walking legs. These are com- 

 paratively smooth and expanded, and obvious teeth are absent from the dactylopodite. 

 This specimen also has dorso-lateral hepatic swellings. 



Acanthonyx macleayi, KRAUSS, 1843 A.I, p. 199. 

 Locality: Cheval Paar, Gulf of Manaar, one ovigerous female, G.I. = 12*50. 



Halimus pleione (HERBST), 1803 A.I, p. 208. 



Localities :- -Pearl banks, Gulf of Manaar, two specimens (ovigerous ? a, c); off 

 Mutwal Island, two specimens (ovigerous ? b and young c? d). 



Description : (a) ovigerous ? . (b) ovigerous ? . (c) ovigerous ? . (d) young $ . 



C.I 17-00 20-00 23-00 11 '00 



C.bi-5-C.L . . . 0-75 0-75 078 0'80 



RL-5-C.L ... [--] 0-37 0-31 0-41 



In the immature male the rostral spines lie in an approximately horizontal plane ; 

 in the three ovigerous females they continue the downward anterior slope of the 

 gastric region of the carapace. 



Halimus hilgendorfi (DE MAN), 1888 A.I, p. 209. 



Localities : Pearl banks, Gulf of Manaar, sixteen specimens (including d and /) ; 

 Aripu coral reefs, Gulf of Manaar, eighteen specimens (including a, 6, c, <gr, and e) ; 

 off Mutwal Island, two specimens (h, i). 



Description: (a) (b) (c) (d) (e) (/) (</) (A) (i) 



ov. ? . ov. ? . ov. $ . ov. ? . ad. g. ad. $ . ad. $ . ad. $. ad. $ . 



C.I 11-50 13-50 14-00 16-50 12'50 12'50 13'50 14'50 15'00 



Rost.spinel.-r C.I. . 0'32 0'44 0'41 0'25 0'52 0'56 0'52 0'52 0'55 

 D. tips K.sp.-i- C.I. 0-20 0-33 0*30 0'23 0'42 0'28 0'40 0'34 0'47 



The above measurements give an indication of the high variability of the length, 

 and degree of divergence, of the rostral spines. Both the characters named are 

 sexually dimorphic. 



A sexual difference is also shown in carapace length. 



Halimus spinosus (A. MILNE-EDWARDS), 1872 A.1, p. 211. 



Locality : Aripu coral reefs, Gulf of Manaar, three specimens (two young ? and 

 young <?). 



Remarks, I unite H. consobrinits, A. MILNE-EDWARDS, and H. spinosus specifically. 

 The slight points of difference are that in the former (l) the anterior angle of the 



