﻿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 branchiae are more distinct in this than in 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. S (PL XII, fig. 1 b) are distinguished from the ordi- 

 nary form of bilobus by the more regular denticulation of the chelae, and by the larger 

 size of the terminal joint. 



Length 1 inch 7 lines, breadth 4 lines ; length of moveable ramus of chelae 

 13 lines. 



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 H inch in length, and the 

 penultimate joint (which is the widest) is 4^ lines in breadth. 



It exhibits the same small intercalated triangular plate at the point of attachment of 

 the dactylos with the propoclos, and the former has at its distal extremity the same 

 minute terminal palette, or unguis, common to the other species of Pterygotm. 



This, together with all the foregoing varieties described in this Part, are from the 

 Uppermost Silurian of Lesmahago, Lanarkshire. 



