THE GENUS PTERYGOTUS 195 
think, however, that it is really a production of the maxillary 
sternum, as its posterior boundary lies behind the level of the 
anterior margin of the base of the mawilla. The free anterior 
extremity of the plate is deeply divided, by a wide median excavation, 
into two lobes, which are provided with many short and fine sete. 
A little, accessory, setose, lobule is also developed from the inner 
surface of the plate on each side. 
A well-developed endophragm separates the sternum of the first 
maxillary, or second post-oral, somite from that of the rest. From 
the mouth to the posterior edge of this, the third post-oral somite, 
the sternal surface slopes gradually downwards; but beyond this 
point, or in the fourth post-oral somite, the sternum makes an abrupt 
projection downwards and then passes backwards, with a general 
parallelism to the axis of the body, for the rest of the extent of the 
cephalo-thorax. 
The antenne are about as long as the proximal nine or ten 
joints of the antennules. They consist of a short basal portion 
supporting two branches of about the same length. The basal 
portion is three-jointed, the inner branch two-jointed, the outer 
multiarticulate. 
Neither antennules nor antenne are chelate or sub-chelate in 
either sex. 
The mandible (fig. 3) consists of a basal joint and a palpiform 
appendage. The outer half of the former is quadrate, convex in- 
feriorly, and presents, posteriorly and externally, a curved articular 
process ; the middle of the basal joint is a little constricted and 
flattened, while its internal portion widens again, and ends in a 
truncated, toothed edge. The teeth are continuous with the blade 
of the mandible, not articulated with it. The palp springs from an 
excavated surface on the anterior and superior face of the quadrate 
outer division. Its proximal joint is short and rounded. The next 
is the longest, and as broad as or broader than the basal joint, 
and wider distally than proximally. At the distal end it supports 
two branches, the outer of which is obscurely five-jointed ; the 
inner two-jointed. The toothed and cutting extremities of the basal 
joints of the mandibles pass between the labrum and the metastoma, 
and bite against one another in the middle line. Those of the 
maxilla lie behind the metastoma. The first maxilla (fig. 4) is 
nearly of the same size as the mandible. Its basal joint is produced 
internally into a large curved process, whose inner edge is beset 
with strong articulated sete, which might almost be called elongated 
teeth, were they not setose along their edges. Succeeding this 
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