220 DESCRIPTION OF THE SPECIES OF PTERYGOTUS 
segments can be seen in our compressed specimen; the former as a 
segment of a circle, the latter as its chord. 
The lateral hinder edges of at least some of the segments are 
produced (fig. 10 g) into short processes, and their margins are acute. 
Six segments appear to be thoracic, and are more transverse 
than the hinder ones, about three times as wide as long (the front 
ones still wider), and each is marked at its hinder extremity by 
two short keels! along the median space, not much raised above 
the surface, and about a quarter of an inch long. The hinder 
abdominal segments, of which fig. 10 g is the first, are destitute of 
these median keels, and gradually longer in proportion to their 
breadth till the tail-joint is reached. The first segment (a) is only 
three-quarters of an inch long by one and three-quarters broad ; the 
second and third (4, ¢) are much narrower, about nine-tenths long, 
The fourth measures more than an inch in length; the fifth one inch 
and a quarter, but rather less in breadth, and narrowed posteriorly. 
It is destitute of any central ridge or keel above or below. The 
tail segment (telson) in this species is a large oval plate, terminated 
by a long apiculus; the length without the apiculus is two inches 
and a half, and with it four inches; the breadth is nearly one inch 
and three-quarters, narrowing to the base, which is thickened, 
(probably cylindrical,) the rest being much depressed. On_ the 
upper surface a strong carina arises near the origin of the joint, 
and continues to the tip of the apiculus. The lower side is flattened, 
or but gently convex at the base. The margin of this tail joint 
is closely serrate or tuberculate in a double line near the base (like 
that of the margin of the head), running into a single line of distant 
long tubercles below and along the apiculus. 
This kind of double tuberculate border is conspicuous down the 
sides of the head, the tubercles being elongated, while on the arched 
front border there are three or four rows. 
The ornamentation of the body rings is quite minute, and is but 
rarely seen in our specimens. It is confined to the anterior border of 
the segments ; the plice are small, equal, and rather prominent. 
Appendages. 
Antenne, Plate II. [Plate 13] figs. 2, 3—The position of these 
serrated organs is settled by their mode of occurrence in the species 
previously described. The fragments, figs. 2, 3, being the largest 
1 The first pair is seen in fig. 10, pushed back over the second, and is the best indication 
of the boundary of the segment a. 
