DESCRIPTION OF THE SPECIES OF PTERYGOTUS 233 
P gigas, Plate VIII. [Plate 19] figs. 6, 7). The proportions also 
of these joints are different from that of the corresponding part in 
Plate V. [Plate 16] figs. 3, 4, being greatly more transverse ; two- 
thirds as long as wide in fig. 1, and about three-fifths in fig. 2. 
This proportion is not very different from that of P. gigas, though 
the shape is so different. May these not be differences of sex, 
Lterygotus Anglicus? or another species. Penultimate body joints, upper and under sides. 
A the upper side, and B the under, showing the anal ridge a. Base of Old Red 
Sandstone, Leysmill, Forfarshire. 
instead of indicating a second species? The ridge (@), supposed to 
be anal, is present in all the species, either subcentrally or on the 
hinder edge of the segment. 
Telson, Plate V. [Plate 16] figs. 5, 6—The tail joint (12th) is 
a broad oval, contracted at its base of insertion, widest fosterzorly 
below the middle, and shortly apiculate at the tip. It is but little 
carinate on the dorsal view, fig. 5, and flat or with a very slight 
median ridge below fig. 6. In a specimen six inches long the 
apiculus does not project half an inch from the genera] margin, which 
is squamate and serrate, but not strongly so. The apiculus itself is 
not serrated. 
