THE GENUS PTERYGOTUS 18I 
its whole length, by only one specimen (Plate VII. [Plate 18] fig. 4), 
and here the distal extremity is rounded and slightly emarginate. 
In the same specimen (and only in this) a fourth joint (7) may be 
observed. It is as long as the third, but of a different form, the 
distal end being much wider than the rod-like proximal extremity. 
The internal edge of the distal extremity is rounded and slightly 
serrated, and is apparently the natural edge of the part; but there 
are appearances which lead one to suspect that the continuation of 
the outer angle has been broken away. 
The characteristic sculpture is visible upon the surface of the 
basal joint of this appendage, more particularly towards the anterior 
edge of its inner region, and the convexities of the facets are here 
directed towards the convex long margin. The outer wall of its 
outer region in fig. 4 is broken away, and the matrix contained in the 
interior has become detached, so as to show the inner surface of the 
opposite wall, which is perfectly smooth. 
In the recent state a considerable interval, doubtless filled by 
muscular and tendinous soft parts, must have existed between the 
two walls, as, even in the compressed fossil, the thickness of the 
matrix filling the cavity sometimes amounts to one-sixteenth of an 
inch. The third and fourth joints also had a considerable thickness. 
(c.) Of the third kind of paired appendages the most complete 
specimens are those figured in Plate VI. [Plate 17] fig. 1, and 
Plate VII. [Plate 18] figs. 1, 2. It consists of an exceedingly 
large and expanded, quadrate, basal joint, produced at one angle 
into a broad curved process, which is obliquely truncated at its 
extremity. The truncated edge is nearly straight, and is serrated, 
broad notches separating a number of strong flattened pointed 
denticles, which are continuous with the substance of the joint, and 
not articulated with it. 
The denticles or serrations form a single series, and diminish in 
size from one end of the series to the other. The smallest is 
succeeded by the rounded corner in which the truncated edge and 
the concave margin of the serrated process meet. 
The surface of the joint, and of its process, is covered with the 
squamiform ornamentation, and presents in the middle of the margin, 
opposite to the serrated process, a deep notch, which receives the first 
joint of the long palpiform remainder of the appendage. 
The form of these joints is particularly described below (p. 236). 
I will only remark here that there are six of them, and that the 
penultimate, much larger than any of the others, is elongated, broad, 
flattened, and widely emarginate at its distal extremity, where it 
