194 ON THE ANATOMY AND AFFINITIES OF 
about equal to the three following ones in length, and forms a 
carapace. The abdomen presents four obvious segments, but the first 
appears to consist of two somites united together, and it is probable 
that the two terminal styliform lobes may represent a somite, in 
which case there would be six somites in the abdomen. The anus 
opens between the bases of the terminal processes. 
The carapace, is not prolonged, either along its posterior margin 
or laterally, but merely overlaps the tergum of the succeeding somite 
in the ordinary manner. Anteriorly, it extends beyond the bases of 
the antennules, and is produced forwards and downwards, between 
them, into two delicate pointed processes, slightly constricted at their 
bases, which are about equal in length to the two basal joints of the 
antennules. The latter are slender, of even thickness, and divided 
into twenty-four setose joints. 
An endophragm, or inflexion of the chitinous integument, dis- 
tinctly separates the sternum of the antennulary, from that of the 
antennary, somite. Viewed laterally, the former appears concave, its 
posterior portion sloping backwards and downwards at an angle 
with the anterior. The anterior half of the antennary sternum, on 
the other hand, is very convex, while its posterior half is produced 
into an abrupt conical protuberance, behind which it passes into a 
broad plate, with a free, convex, posterior edge. This projecting mass 
developed from the antennary sternum appears to me to represent 
both the epistoma and the labrum of decapod Crustacea. 
Viewed from below (Plate XVI. [Plate 27] fig. 2) the part 
described has the aspect of a very broad, almost triangular plate, 
occupying nearly the whole width of the antennary sternum, and 
presenting a prominent median portion and two lateral ale, which 
slope away backwards and upwards. The posterior portion of the 
median lobe is rounded and somewhat expanded at its extremity, and 
the lateral edges of the expanded end are provided with fine sete, 
which extend over its superior face for a short distance. This con- 
joined epistoma and labrum forms the anterior boundary of the oral 
vestibule ; its inner face is beset with short sete, and a short median 
tooth, rounded at its extremity, projects backwards in the middle 
line. 
Posteriorly, the oral vestibule is bounded by a quadrate plate, the 
metastoma, directed forwards and downwards, and having about 
two-thirds of the width of the labrum. As the sterna of the two 
somites which immediately succeed the mouth, viz., the mandibular 
and the first maxillary, are not distinctly separated from one another ; 
it is difficult to say from which this plate arises. I am inclined to 
