﻿PLATE XI. 



UPPER SILURIAN CRUSTACEA. 



Order — Merostomata. 



Sub- Order — Eurypterida. 



Fig. 



1. Pterygotus bilobus, var. (3, crassus. 

 o, o. The compound eyes. 



s. The serrated mandibular border of one of the ectosnaths. 

 ec, ec. The ectognaths, preserved in situ, 

 e. One of the endognaths, displaced. 

 a. A chelate antenna ; the claw or pincer hidden beneath the telson. 



2 a. Seven of the anterior segments of Pt. bilobus var. y, peromatus (== var. plicatissimus, 

 Salter). 



op. The opercular plate, squeezed forward and much distorted. 

 c. The central appendage of same. 



br. The branchiae exposed to view, protruding from beneath the upper 

 and originally attached border of the operculum. 



2 b. The above described portion of 2 a, magnified twice (the letters are the same as in 

 fig. 2 a, above). [The direction of the plicse upon the operculum indicates that the 

 lower edge in our Plate was that by which it was originally attached to the pos- 

 terior border of the under side of the head. The head, and probably the first two 

 body-segments, are wanting.] 



3. Detached antennas of a very large Pterygotus (probably Pt. bilobus, var. y). 



ch. The moveable chela, with its large teeth, resembling those of Pt. anglicus 

 (see Part I, PI. VII, figs. 1—3). 



The surface at b is covered with the valves of Beyrichia Kloedeni 

 or gibba (a small bivalved Entomostracan), a pair of which are 

 enlarged and figured at b 1. 



Pigs. 1, 2 a, and 3, are drawn of the natural size; fig. 2 b magnified twice; fig. b 1 

 magnified six times. 



From Uppermost Silurian of Lesmahago, Lanarkshire. 



Pigs. 1 and 3 are in the British Museum. Fig. 2 is in the Museum of Practical 

 Geolosjv, Jermvn Street. 



