THE GENUS PTERYGOTUS I9gI 
identified as a part of a Pterygotus. It is a broad quadrate plate, 
abruptly truncated on two of its opposite sides, one of which is much 
longer than the other, and having the other two margins equal and 
rounded. Two folds or ridges run from one truncated edge to the 
other, and divide the plate into three lobes, a median and two lateral. 
The median lobe presents a series of curved lines or ridges disposed 
symmetrically on either side of the median line. Similar folds in the 
one lateral lobe are. more or less symmetrically arranged with regard 
to those on the other lobe. 
The shorter truncated margin of the plate presents two impres- 
sions, like the remains of articular surfaces, and there are similar 
impressions at the outer extremities of this margin. Of these, 
that on one side gives attachment to a long jointed appendage 
presenting six distinct articulations. The basal joint is short and 
much wider than the others. The distal edge of the fourth presents 
a series of strong spines, the sixth has the form of a curved claw. 
I know of only two crustacean structures with which this body 
can be compared, the one is a carapace, the other the swimming limb 
of a copepod with its coalesced, lamellar, basal joints greatly developed 
(compare, Plate XVI. [Plate 27] fig. 7). 
Systematic Position of Pterygotus—In comparing Péterygotus with 
other crustaceans, for the purpose of determining its systematic position 
and relations, several of the largest and best defined orders of the 
Crustacea may at once be left out of consideration. Prerygotus is 
clearly not one of the 7r¢lobita, Cirripedia, Ostracoda, Edriophthalinia, 
or Stomapoda (if we restrict this order to these Crustacea which have 
pedunculate eyes, and distinct and movable ophthalmic and anten- 
nulary somites). Nor are there any known Branchiopoda to which 
these great extinct crustaceans have relations of affinity, so that 
if they are to be referred to any existing order, it must be 
either that of the Podophthalmia, that of the Copepoda, or that of 
the Pecilopoda. 
The first step towards determining the systematic place of Ptery- 
gotus, therefore, is to consider the reasons for and against the assign- 
ment of a position in either of these orders to them. 
To those who are acquainted only with the ordinary Podoph- 
thalmia, the discussion of the affinities of Pterygodus, with forms in 
every way so distinct, may seem superfluous ; and there is assuredly 
little enough in common between a crab or a shrimp and their 
palzozoic congeners; but there are one or two sections of the 
Podophthalmia which, while they still hold fast by the typical 
characteristics of their order, become so modified in many important 
