376 CEYLON PEARL OYSTER REPORT. 



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

 R.l.-M '.]. has mean value = 0'47, and range of variation from - 32 to 0'59. For the 

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



Specimen s (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 lias dorsolateral hepatic swellings. 



Acanthonyx macleayi, Krauss, 1843 A. 1, p. 199. 

 Locality: Cheval Paar, Gulf of Manaar, one ovigerous female. ('.1. = 12"50. 



Halimus pleione (Herbst), 1803 A. 1, p. 208. 



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

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



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



C.l 171)0 20-00 23-00 111)0 



C.b.-r-C.l. . . . (i-75 0-75 0-78 0-80 



R.L-S-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 hilg-endorfi (de Man), 1888 A. 1, p. 209. 



Localities : Pearl banks, Gulf of Manaar, sixteen specimens (including <1 and /*) ; 



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



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



Description : () 



ov. ? . 



Gl 11-50 



Rost.spinel.-^C.l. . 0"32 

 D. tips R.sp.-=-C.l. 0-20 



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 V and 

 young <?). 



Remarks. I unite H. consobrinus, A. Milne-Edwards, and H. spinosus specifically. 

 The slight points of difference are that in the former (1) the anterior angle of the 



