52 A HISTORY OF RECENT CRUSTACEA 
plate, the coalesced ventral plates of the last five seg- 
ments of the trunk are distinguishable, and three earlier 
segments are obscurely represented. It is never entirely 
linear. The vulve of the females are generally placed 
upon it, but in some groups are transferred to the basal 
joints of the ante-penultimate legs. The pleon is of sub- 
ordinate size, usually reflexed against the concavity of the 
plastron, in the male generally narrow and pointed, with 
only one or two pairs of pleopods, in the female broad, 
with four pairs of pleopods. ‘The basal joint of the first 
antennz contains auditory hairs but no otoliths. 
In this definition the Anomura apterura are included. 
To the dry bones of definition must be added an even less 
appetising explanation of terms in common use for the 
description of genera and species. The orbital regions of 
the carapace speak for themselves as being those which 
contain the eyes. The ‘ front’ lies between them. Behind 
it on the under surface are the fossettes of the first an- 
tenne, followed in the median line by the epistome, the 
buccal or oral frame, and the sternal plastron. The second 
antennz are placed outside of and a little behind the first. 
The ‘hind margin’ of the carapace separates the trunk 
from the pleon, and lies between the first joints of the last 
pair of trunk-feet. Between it and the orbits are the 
lateral margins, each of which is subdivided into an 
antero-lateral and a postero-lateral portion forming, when 
not continuous, the epibranchial angle. The dorsal sur- 
face of the carapace is marked by several grooves cor- 
responding with the insertions of muscles underneath, and 
also forming the boundary lines of regions which roughly 
coincide with the positions of important internal organs. 
Along the centre lie the gastric, cardiac, and intestinal 
regions, respectively over the stomach, heart, and intes- 
tine. The hepatic regions over the liver flank the gastric 
region on either side in front, and behind these le the two 
branchial regions, the ‘ cervical groove’ being that which 
separates the gastric and hepatic regions from the cardiac 
and branchial. On the under side the pterygostomian re- 
gions, ‘the wings of the mouth,’ lie between the antero- 
