204. DESCRIPTION OF THE SPECIES OF PTERYGOTUS 
large rounded eyes and wide expanded penultimate body joint in- 
dicate that the species belongs to Section 2. The forward position 
of the eyes is extravagantly shown in such species as P. acumdnatus, 
in which, too, the telson attains its greatest degree of elongation. 
P. stylops has a similar but even more prominent eye. 
The overlapping of the basal joints of the swimming feet in 
Section 1, and the attachment of the other joints lower down, 
may be characters of less importance, but they are associated in 
P bilobus and P. tnornatus with the characteristic elongated eyes, 
and the central Jobe of the labrum is ovate, instead of sagittate 
in these species. The terminal joint or telson is also of like shape in 
P. bilobus and P. Banksii, the only two species of the sub-genus in 
which it is known. 
Although, therefore, Héimantopterus has been restored to Pétery- 
gotus, the character in which it was formerly supposed to differ, viz., 
the lateral eyes, being common to both, it is advisable to keep the 
above two sections or subgenera distinct; and it may hereafter be 
necessary, when we have a more complete knowledge of the several 
species, to reinstate the section as a genus, in which case the name 
Erettopterus' will be appropriate. It is certainly more remote from 
Eurypterus than is Pterygotus proper, and some unpublished species 
of the former have the eyes so far outwards as to make a near 
approach to the section or subgenus here described. The shape of 
the antenne, however, is quite enough to separate them; and the 
telson of Eurypterus, so far as it is known, is linear, or linear 
lanceolate. 
Pterygotus is known now to range from the Upper Llandovery 
Rock (or May Hill Sandstone) through all the overlying Upper 
Silurian Rocks into the Cornstones of the Old Red Sandstone, 
though it is but rare above the base of the last-named formation ; 
its metropolis seems to be at the point of transition between the 
Silurian and Devonian formations. Fifteen species are known. 
Eurypterus began not quite so early, being first met with in the 
Upper Ludlow Rock, and thence, becoming abundant in species 
(though of small dimensions) in the passage beds at the base of the 
Old Red Sandstone, attained its maximum of size in the upper 
portions of that formation and the base of the carboniferous rocks, 
after Ptervgotus had apparently ceased to exist. (See Quart. Journ. 
Geol. Soc. for 1859, vol. xv. ined.) About sixteen species are known. 
1 épértw, for épécow, to row ; instead of Ffimantoplerus ; see note p. 175. 
” Stylonurus, Page, has a similar form, but possesses thirteen body segments and long 
linear swimming feet. 
