182 ON THE ANATOMY AND AFFINITIES OF 
articulates with an oval palette-like plate, with serrated edges. A 
brief comparison of the figures will satisfy the reader that this is the 
part represented by Professor Agassiz in the lower right-hand corner 
of Plate A (doe. c?z.). 
From the form of the articulating edges of the joints of the 
palpiform part of this appendage, I am inclined to think that, as 
in the chela of the lobster, the plane of motion of each joint formed 
a considerable angle with that of its predecessor and successor, the 
result of which would be a sort of feathering, or screw-propeller 
motion, of the ultimate and penultimate joints during flexion of the 
limb. 
The Metastoma—tThe last kind of appendage (Plate VI. [Plate 17] 
fig. 7) | have to describe is perfectly symmetrical, and hence, even 
if there were no other means of determining its real nature, its single 
or azygos character might be divined 
It is an oval plate with margins much thinner than the centre, 
and presenting a median notch at one extremity. It is richly 
sculptured, the facets having their convenities turned towards the 
rounded entire end; and it is worthy of notice that the facets are 
singularly close set around the apex of the emargination. It is as 
if the plate had been once completely oval and evenly ornamented 
and had then been folded in at the emargination. 
The part represented in Plate VII. [Plate 18] fig. 7, closely 
resembles the basal joint of the appendage described under the head 
(é) in general aspect, but differs from it in the excavation of the 
more convex margin, in the length of the latter, and in the fact that 
the longest tooth arises considerably behind the junction of the 
curved with the truncated margin. If these appearances do not 
arise from mutilation or other alteration, this part is probably a fourth 
paired appendage. 
The carapace of Prerygotus anglicus has not been found in con- 
nexion with the body, nor the epistoma with the carapace, nor any 
of the appendages with any other part of the body. A_ peculiar 
value consequently attaches to those entire specimens of a crusta- 
cean formerly denominated Azmantopterus, but which the further 
information yielded by the extensive materials on which this Mono- 
graph is based, shows to be of doubtful generic distinctness from 
Pterygotus. 
A well-preserved specimen of P. (Aimantopterus) bilobus is 
represented in Plate I. [Plate 12] fig. 1. It presents an elongated 
body, rounded anteriorly and posteriorly, and much narrower in its 
posterior, than in its two anterior, thirds. Assuming that the dorsal 
