68 BRITISH FOSSIL CRUSTACEA. 
operculum and the anterior border of the foremost body-segment there are preserved 
undoubted remains of branchial plates, figs. 2a and 2 4, dr, dr, presenting the same 
delicate leaf-like vascular appearance as is seen in the specimens figured in Pls. XII 
and XIII. 
These delicate membranous plates in Pterygotus differ in form from the corresponding 
organs in Limulus ; but there is no doubt they occupied the same relative position. In 
Limulus the vascular striz are parallel to the circumference of the lamella ; in Pterygotus 
they appear to have branched and subdivided from the centre to the margin of the plate, 
becoming finer and more numerous towards the border. 
As I shall have occasion to refer again to the branchie in the genus S/imonia, I do 
not propose to enter more fully upon their description here, especially as I hope shortly 
to have executed some carefully prepared figures of the respiratory organs of Limulus 
and other recent lamellibranchiate Crustacea for illustration and comparison. 
In the details of the ornamentation and in the form of the body-segments this 
specimen (PI. XI, fig. 2a) agrees with that figured in Pl. XII, fig. 1 a. 
Var. 4.—PreryGotus BILOBUS, var. 8, acidens. PI. XII. 
This specimen, to which we have had occasion already to refer when speaking of the 
branchize of Pterygotus, is full of interest. It is preserved in the Museum of Practical 
Geology, Jermyn. Street. 
We see the head, with its great compound eyes (0, 0), and its lateral eye-spots (/ e), its 
chelate antenna (a), its great ectognaths or swimming-jaw-feet (ec), its metastoma or 
post-oral plate (m); the thoracic plate or operculum (oy), with its branchiz (47); and 
all the segments present (save 15 and 16, which are indicated with a lighter shading), 
including the broad bilobed telson (20). 
The general outline of the body is globose, probably more so in proportion to its size 
than any example of var. perornatus. The dimensions are as follows :— 
