A STUDY IN CARCINOLOGY. 47 
mandible, thus forming the hinge-line about which the mandible rotates. 
The postero-external edge of the plate is ill-defined and passes into an f 
imperfectly calcified area forming the roof of the branchial canal. The 
uiembrane narrows posteriorly, and arriving at the external articular pro- 
cess of the mandible, again becomes calcified and thickened to form a rather 
large strophidiuin for articulation with the rnandible. After comparison 
with many other species, I have satisfied myself that the whole of this area, 
calcified in front and behind, but imperfectly calcified or membranous in 
the middle, represents the epimeron of the mandibular segment, which 
in the Astacura is fused to the antennary epimeron, but in some primitive 
Brachyura, as I shall show subsequently, is quite independent of it and 
also of the antennary sternum. Relying on these interpretations which, I 
■venture to think, will be accepted by anyone who makes a careful compara- 
tive study of this region, I shall henceforth discard the misleading and 
purely topographical names in general use and speak of the sterna, epimera, 
etc., of the various segments. 
Before bringing the Raninidse into comparison it will be well to consider 
the characters presented by some other Oxystome crabs, particularly of 
the Dorippidse from which, according to some authors, the Raninidse are 
descended. I have studied Dorippe lunata, M. Edw., yEthusa c'diatifrons. 
Fax., and ^thusina gracilipies, Thiers. Of these the last-named species 
shows the most primitive and interesting features, and its mouth-frame and 
antennary region are depicted in fig. 21. The basal joints of the antennules 
are enormously inflated and interposed between the exposed portions of the 
ocular peduncles and the rostrum. The antennas have a four-jointed peduncle 
and a fairly long flagellum ; the basal joint is small but freely moveable. 
The downward process of the front or rostrum articulates with the antennulary 
sternum, the hitter being produced right and left into wings which are 
adapted to the. shape of the inflated basal joints of the antennules. In this 
species the antennulary sternum is largely exposed, only its posterior margin 
is overlapped by the front end of the spout-shaped antennary sternum. 
The latter sclerite is well defimed and the sutures uuiting it to adjacent 
parts can be seen clearly. It consists of a central somewhat scutiform plate 
with antero-external projections of which the edges are inrolled ventrally to 
form the spout-shaped Oxj^stomatous mouth. It is to the hinder ends of 
these lateral infolded projections that the pterygostomial processes of the 
carapace are united. Behind the body of the antennarj' sternum is the 
manilibular sternum, a very slender but distinct and calcified curved bar. 
Right and left, intervening between the lateral wings of the antennary 
sternum and the outer ends of the mandibular steimuni, are two distinctly 
defined oval plates forming the roof of the anterior part of the branchial canal 
and extending so far backward that they supply the articular strophingia for 
