14.0 A HISTORY OF RECENT CRUSTACEA 
so, though such a species as Petalomera pulchra in the pre- 
ceding legion does not seem to suit the theory. One may, 
however, suppose that in some instances a re-widening of 
the plastron may have been developed without any re- 
arrangement as to the position of the vulve. 
This legion contains the single family Raninidee, for 
which, therefore, no separate character is needed. It 
includes some nine genera, limited to the warm seas, and 
inhabiting chiefly the tropics, with a range of depth 
apparently not exceeding 300 fathoms. 
Ranina scabra (Fabricius), originally called Cancer 
ranimus, Linn., and afterwards Ranina serrata, Lamarck, 
and Ranina dentata, Latreille, from Amboina and the Sand- 
wich Islands, was known to fame long before a separate 
genus was established for it. The carapace has been com- 
pared to an inverted triangle. It is very broad anteriorly, but 
the sides slope very gradually to the rounded hinder margin. 
The eye-stalks are three-jointed, strongly geniculate, and 
have a very deep orbit. The pterygostomian regions of the 
carapace unite with the sternal plastron so as completely 
to separate the third maxillipeds from the chelipeds. The 
plastron itself 1s anteriorly almost trefoil-shaped, but to 
the rear becomes linear. The branchiz, Milne-Edwards 
says, are arranged as in the Brachyura, but in the confor- 
mation of the respiratory cavity there is a peculiarity of 
which he knew no other example. As in the Leucoside, 
the carapace is joined to the sternum and the cavity of the 
sides, without leaving above the base of the feet or maxil- 
lipeds any space for the entrance of the water necessary 
for breathing, but the afferent channel instead of being 
pierced beside the efferent channel, on the sides of the 
mouth, is situated behind and has a special opening below 
the base of the plecn. This view, however, is criticised by 
de Haan in a passage that is not free from perplexity. 
‘In Portunus and Grapsus,’ he says, ‘the water is brought 
to the branchize by a double path and removed by a double 
path ; it reaches the branchial cavity by the mouth and 
the apertures near the base of the chelipeds; but it passes 
out both by the space between the inferior lateral margin 
