A STUDY IN CARCINOLOGY. 55 
expanded, and modified to form the operculifovm floor of the exhalant canal. 
The exopodite is somewhat modified and to a varying extent in different 
genera, but it is alwaj^s shorter than the endopodite, does not share in the 
formation of the operenluin, on the whole retains the characteristic shape of 
an exopodite, and invariably bears a flagellnm — I hope I may not be criticised 
for attaching undue importance to a trifling character. It is just sucli 
instances as this, in which the same purpose is effected by somewhat different 
means, that afford the best criteria as to whether some particular structural 
resemblance is homogenetic or homoplastic. It was the discovery of this 
difference in the structure of the first maxillipeds that finally confirmed my 
opinion that the Raninidse are not genetically connected with the other 
Oxystomatous Grabs. My reliance on this, item of evidence was somewhat 
shaken by A. Milne Edwards and Bouvier's statement that the opercular 
floor of the exhalant canals in Cymonomus is formed by the dilated exopodites 
of the first maxilliped, but an examination of their figure (28, pi. xv. fig. 6 — 
there is a mistake in the numbering of these figures — ) shows that the 
resemblance to the Raniuidfe is only superficial, and the exopodite of the first 
maxilliped of Cymonomus has a long six-jointed flagellum furnished with long 
setse. My test case, therefore, holds good, and the evidence produced is 
sufficient to justify Boas' doubt as to the inclusion of the Eaninidae among 
the other Oxystomata, and A. Milne Edwards and Bouvier's decision to 
exclude them from their monograph on the Oxystomata of the ' Blake ' and 
'Hassler' Expeditions. The Eaninidaj, therefore, must be removed from 
their present systematic position and assigned to some other place in classi- 
fication. They cannot be excluded from the section Brachyura for reasons 
given on p. 27, but in conformity with more recent and generally accepted 
classifications they must rank as a separate tribe, equivalent to the Dromiacea, 
Brachygnatha, and the rest of the Oxystomata. For this new tribe I propose 
the name Grynmopleura*, which directs attention to one of the most charac- 
teristic features of the group, namely the exposure of the epimera of the 
posterior thoracic segments. This new tribe may be defined as follows. 
Tribe GYMNOPLEURA. 
Anterior thoracic sterna broad, posterior thoracic sterna narrow and keel- 
like ; posterior thoracic epimera largely exposed by reduction of branchio- 
stegite ; female openings on coxae ; last pair of pereiopods dorsal in position, 
* Mr. X. a. R. Stebbing in ' Nature,' Jany. 19:32, liaa pointed out that Latreille (Cours 
d'-Entomologie, 1831) iustituted the tribe Notopterygia for Ranina. At first sight it seemed 
advisable to adopt Latreille's name for the tribe coraprisiug the Kaniuidse instead of my 
own. ]3ut tlie rules of nomencliiture do not apply to tribal names; Latreille's sj'stem of 
classiiicatiou differs largely from that followed in this paper and his reasons for placing 
Ranina in a separate tribe differ widely from mine. Confusion rather than perspicuity 
would result from the adoption of Latreille's tribal name. 
