70 BRITISH FOSSIL CRUSTACEA. 
There is little doubt but that we have in this species evidence of a second and more 
membranous thoracic plate lying within the outer one, but its border is but ill defined 
in our plate. 
The branchize are more distinct in this than m any specimen I have seen. 
The largest leaflet is 8 lines in length by 4 lines in breadth ; the smaller are 4 lines 
in length and 2 lines wide. 
The chelate antennules in var. 6 (Pl. XII, fig. 1 4) are distinguished from the ordi- 
nary form of dc/obus by the more regular denticulation of the chelz, and by the larger 
size of the terminal joint. 
Length 1 inch 7 lines, breadth 4 lines; length of moveable ramus of chele 
13 hnes. 
The metastoma, or post-oral plate, does not appear to be truncated posteriorly, as in 
var. perornatus, and is somewhat more oval and less elongated; and the emargination of 
the anterior border is much more slight. 
Length 15 lines, breadth 10 lines. 
Both ectognaths are preserved entire, and the serrated edges of their great basal joints 
are seen pressing through the head-shield, just in front of the larval eye-spots. The 
free swimming-foot attached to the coxal joint measures 14 inch in length, and the 
penultimate joint (which is the widest) is 43 lines in breadth. 
It exhibits the same small intercalated triangular plate at the point of attachment of 
the dactylos with the propodos, and the former has at its distal extremity the same 
minute terminal palette, or unguis, common to the other species of Pterygotus. 
This, together with all the foregoing varieties described in this Part, are from the 
Uppermost Silurian of Lesmahago, Lanarkshire. 
