52 A HISTOEY OF EECENT 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 vulva3 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 

 antennae contains auditory hairs but no otoliths. 



In this definition the Anomnra apterura are included. 

 To the dry bones of definition must be added an even less 

 appetising explanation of terms in common use for tbe 

 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- 

 tennas, followed in the median line by the epistome, the 

 buccal or oral frame, and the sternal plastron. The second 

 antennae 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 lie 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- 



