THE GENUS PTERYGOTUS 201 
twelve segments behind the carapace (though only eleven are repre- 
sented in his figure). Two pairs of slender jointed appendages are 
figured both by him! and by Harlan in front of the ectognaths ; but 
no chelate antenna are represented, and the connexions of the 
jointed appendages are not made out. The characteristic sculpture 
is well exhibited by Aurypterus. Even if the chelate antennz are 
really absent in Eurypferus, it must be remembered that these are 
organs particularly liable to variation among even closely allied genera 
of Copepoda. 
Leaving the determination of the precise differences between the 
Pterygotus and Eurypterus to future investigators, their resemblances 
to one another, and their common differences from all other 
Crustacea, are sufficiently important to justify their association into 
a distinct order—the “ Eurypterida,’ which may be thus defined: 
Crustacea with numerous free thoracico-abdominal segments, the 
penultimate, and probably all the rest of which, are devoid of appen- 
dages; with the anterior somites united into a carapace, bearing a 
pair of large marginal or subcentral eyes; with a very large epistoma 
and metastoma ; with three, or four pair of moveable cephalic appen- 
dages, the posterior of which form great swimming feet, and with the 
integument characteristically sculptured. 
Of the mode of life of these extinct Crustacea we know nothing, 
but we may conjecture that it was their habit sometimes to creep 
along the bottom of the waters which they inhabited, like the 
Limulus of the present day, sometimes to propel themselves by the 
rapid flexion of their great swimming feet, aided perhaps by the 
sudden extension of their free segments. 
No existing or extinct crustacean has so massive a body as 
Pterygotus, some species of which there is every reason to believe 
attained a length of at least five feet; but mass, in an active animal, 
involves large muscles, and these require solid points a@'appur. Hence 
we may conclude that the integument of the Prerygotus, thin and 
fragile as are its remains, possessed a great amount of firmness in the 
recent state. 
That the singular thinness of the fossil test, constituting a mere 
joints of the ectognaths, while the middle ovate ‘under lip” has just the position and general 
form of the metastoma. Add to these the triangular ‘‘upper lip,” and the resemblance to 
Prterygotus becomes not a little striking. 
1 Roemer states that his Zurypterus also had twelve free segments behind the carapace. 
The eyes borne by the latter were uniform and not faceted; only two appendages, both 
on the left side, remained. The posterior had the general structure of the ectognathary 
limb of Pterygotus. The anterior is slender, four-jointed, and perhaps terminated in a 
chela. 
