THE GENUS PTERYGOTUS 187 
The latter is four-jointed, the proximal articulation being large, 
quadrate and setose along its anterior margin, and provided with 
a large and strong curved spine towards the outer extremity of that 
margin. The next joint is elongated and curved, its posterior 
margin being slightly convex, the anterior similarly concave, and 
towards the distal extremity bearing a curved spine, like that in the 
preceding joint. 
The third joint and the fourth are much shorter. The former 
carries a spine on its anterior edge. The latter has two spines, one 
anterior and one terminal. Is this the homologue of that endognath 
of Prerygotus anglicus represented in Plate VII. [Plate 18] fig. 4, 
or does it, as the structure of its basal joint would seem to indicate, 
correspond with the apparently different part shown in fig. 7? 
2. Plerygotus acumtnatus.—On the same slab with a large portion 
of the body and one of the ectognaths of this species, is seen the 
remains of an appendage whose basal joint clearly resembles the 
corresponding part in P. anglicus (Plate XIII. [Plate 24] figs. 2, 3). 
It consists of a broad, flattened, quadrate plate, having a series of 
curved and striated teeth articulated with its well-defined, nearly 
straight edge. These teeth, however, do not form one even series, 
either as regards their size or their position. They commence large, 
near to what I will term the anterior margin of the plate, but 
not close to it, the antero-lateral angle being rounded off and 
giving rise to no tooth. In this respect it represents the corre- 
sponding part of the endognath of P. punctatus just described. 
After four or five large ones, the teeth rapidly diminish in size and 
become indistinguishable. The free edge of the plate then makes a 
slight curve, so that it projects beyond its former line and then gives 
rise to two teeth of the same size as the most anterior ones, after 
which it becomes lost beneath the matrix. A depression runs from 
the commencement of this projecting edge into the crushed and 
transversely folded, middle part of the plate. In front of this 
crushed portion, the anterior region of the plate is, in its outer half, 
richly sculptured ; behind and internally, the substance of the organ 
is for the most part broken away, but shows the remains of a similar 
sculpture outside the broken edge. The opposite half of the specimen 
exhibits the impression of the sculpture over this broken part, but in 
neither half can the ornamentation be satisfactorily traced continuously 
over the middle crushed region. 
The outer part of the joint, just described, is continued into a 
broad mass, so divided into two portions externally, as to appear 
like two palps. Each palp-like division appears to possess at least 
