IgI0. | A. ALCcocK : Classification of the Potamonidea. 261 
Subfamily TRICHODACTYLINAS. 
This subfamily is South American, straggling into Central 
America. 
To me, as to Dr. Calman, its present position in the system 
is not altogether satisfactory. 
There is no question that the members of this subfamily are 
very remarkably different from all other Potamonide, in many 
respects: the dactyli of the crawling-legs are devoid of the charac- 
teristic spines ; the merus of the external maxillipeds is quite a 
long joint ; the postero-lateral borders of the carapace are sharply 
defined ; and in several species the middle segments of the parti- 
cularly broad male abdomen are fused. Differences so numerous, 
and (for Cyclometope crabs) so great, appear to me to indicate a 
different ancestry. 
a ne et 
