38 BRITISH FOSSILS. 



indicate that the species belongs to Section 2. The forward position 

 of the eyes is extravagantly shown in such species as P. acuminatus, 

 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. inornatus with the characteristic elongated eyes, 

 and the central lobe of the labruin 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, Himantopterus has been restored to Ptery- 

 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 Eurypterus than is Pterygotus proper, and some unpublished species 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 antennae, however, is quite enough to separate them ; and the 

 telson of Eurypterus, so far as it is known, is linear,f or linear 

 lanceolate. 



Pterygotus is known now to range from the Upper Llandovery 

 Hock (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 Pterygotus had apparently ceased to exist. (See Quart. Journ. 

 Geol. Soc. for 1859, vol. xv. ined.) About sixteen species are known. 



* tpfTTia, for fyfWw, to row ; instead of Himantopterus^ see note p. 10. 



f Styloiiurus, Page, has a similar form, but possesses thirteen body segments and long 



ear swimmin feet 



linear swimming feet 



