218 WALTER GARSTANG. 



a continuous relative reduction of the water-supply for the 

 branchiae, and consequently of the area occupied by the aflferent 

 current. 



Utility of Specific Characters in the Genus Calappa, 



The most recent revision of the genus Calappa, that of 

 Alcock (1896, pp. 140 — 148), although confined to the nine 

 species found on the Indian coasts, shows that the characters 

 which are employed to discriminate the different species are 

 principally the following : 



1. Proportion of length to breadth of carapace. 



2. Extent of postero-lateral clypeiform expansions of 

 carapace. 



3. Serrations of carapace margins : 



i. Antero-lateral margins, 

 ii. Margins of clypeiform expansions. 



4. Hairiness of pterygostomial regions. 



I make no pretence to be able to explain the possible utility 

 of the varied combinations of these features which the different 

 species of Calappa present, when my only material is the 

 literature upon the genus and some spirit specimens of several 

 species. Nevertheless I venture to point out certain correlations 

 which are not without their significance in this connection. 



1. On the whole the more elongated species are restricted 

 to deeper water than the broader species. 



2. The clypeiform expansions are largest in the shallow- 

 water species and smallest in those which inhabit deep water ; 

 cf. small size of expansions in C. pustulosa (25 fms.), C. 

 Woodmasoni (34 fms.), and C. exanthematosa (100 fms.). 



This correlation is confirmed by the fact that the species of 

 the allied genus Mursia, "which is practically Calappa 

 without the wings to the carapace," are found exclusively in 

 the deep sea (e.g. M. bicristimana, 150 — 400 fms.). 



3. The denticulations of the antero-lateral margins I have 

 shown to subserve a sieve-like function in British crabs with an 

 elongated orifice to the exostegal canal. It is probable that 



