PLATH SX 
UPPER SILURIAN CRUSTACEA. 
Order—MEROSTOMATA. 
Sub- Order-—HuRYPTERIDA. 
Fie. 
1. Pterygotus bilobus, var. B, crassus. 
o, 0. ‘The compound eyes. 
s. The serrated mandibular border of one of the ectognaths. 
ec, ec. The ectognaths, preserved in siti. 
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 P¢. dilodus var. y, perornatus (= var. plicatissimus, 
Salter). 
op. The opercular plate, squeezed forward and much distorted. 
c. The central appendage of same. 
ér. The branchize exposed to view, protruding from beneath the upper 
and originally attached border of the operculum. 
2 4. The above described portion of 2 a, magnified twice (the letters are the same as in 
fig. 2 a, above). {The direction of the plicee 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 antenne of a very large Pterygotus (probably Pt. bclobus, var. y). 
ch. The moveable chela, with its large teeth, resembling those of Pt. anglicus 
(see Part I, Pl. VII, figs. 1—3). 
The surface at B is covered with the valves of Beyrichia Kledeni 
or gibba (a small bivalved Entomostracan), a pair of which are 
enlarged and figured at B 1. 
Figs. 1, 2 a, and 38, are drawn of the natural, size; fig. 2 6 magnified twice; fig. B 1 
magnified six times. 
From Uppermost Silurian of Lesmahago, Lanarkshire. 
Figs. 1 and 8 are in the British Museum. Fig. 2 is in the Museum of Practical 
Geology, Jermyn Street. 
