ON THE ANATOMY AND AFFINITIES OF PTERYGOTUS 171 
only in 1840 that I obtained direct proof that such was the case. In 
fact, among the specimens collected and communicated to me at that 
time by Mr. Webster, are fragments of carapaces, segments of the 
tail, the natatory palettes with which the extremity of the latter 
was provided, legs, and chela. These remains remove al] doubt as 
to the zoological position of the fossil. It is a crustacean of colossal 
size, having a carapace of more than a foot and a half in width, and 
a tail about a foot wide. The dimensions of the cephalothorax, repre- 
sented in the right-hand figure of the second row, Plate A,! forbid 
its arrangement among the Decapoda, notwithstanding the form of 
the chela figured in the middle of the lower series of the same plate.” 
I am rather inclined to believe that this singular animal will 
become the type of a family, intermediate between the Trilobites 
and the Eztomostraca, in which perhaps the Aurypter? and the 
Eidothee will some day be included. The carapace is ornamented 
throughout with a squamose sculpture, which confers upon its surface 
the aspect of a fish’s cuirasse; in the middle it presents an im- 
pression like that of a lance-head, corresponding, without doubt, 
to the gastric and cardiac regions of ordinary Crustacea. The 
divisions of the chela are provided with large obtuse teeth ; the 
longer point of these levers is strongly arcuated. The arm which 
carried the chela® (figured on the left in the lower series) is very 
large; the articulations which immediately precede the pincer are 
short, and wider than they are long. The ordinary legs (figured 
on the left in the middle series*) are simple, and end in a point ; 
the segments to which they are attached are sensibly longer than they 
are wide. 
“The segments of the tail® (represented in the right-hand figure 
of the upper series,) reduced to half their natural size, are broad 
plates, provided with articular processes at their superior extremities. 
Lastly, the natatory palettes of the tail (the two figures at the 
superior left-hand and inferior right-hand corners®) are rounded 
plates, fringed along their edges, and scaly on the surface, like the 
carapace. This curious fossil does not seem to be very rare in the 
1 This is in fact the epistoma. 
? The part thus figured is one of the antenne. 
3 The figure here referred to represents a part of the palp of the ectognath, which has 
nothing to do with the chela. 
+ This is an endognath with its palp; no segment is represented in the figure. 
5 The figure represents one of the most anterior segments of the body. 
6 The part shown in the superior left-hand figure is part of the basal joint of an 
ectognath. The inferior right-hand figure represents the terminal joint of the palp of the 
same organ. 
