﻿68 BRITISH FOSSIL CRUSTACEA. 



operculum and the anterior border of the foremost body-segment there are preserved 

 undoubted remains of branchial plates, figs. 2 a and 2 b, br, br, presenting the same 

 delicate leaf-like vascular appearance as is seen in the specimens figured in Pis. 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 striae are parallel to the circumference of the lamellae ; 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 branchiae in the genus Slimonia, 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. 2 a) agrees with that figured in PI. XIII, fig. 1 a. 



Var. 4. — Pterygotus bilobus, var. S, acidens. PI. XII. 



This specimen, to which we have had occasion already to refer when speaking of the 

 branchiae of Pterygotus, is full of interest. It is preserved in the Museum of Practical 

 Geology, Jermyn Street. 



We seethe head, with its great compound eyes (o, o), 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 (op), with its branchiae (br) ; 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 : — 



