116 BRITISH FOSSIL CRUSTACEA. 



appear to be arranged in linear series, and were attached in single or double rows to the 

 under surface of the body by their upper end, whilst their lateral and lower rounded 

 leaf-like borders were freely bathed in the fluid medium in order to oxygenate the 

 blood. 



These leaf-like appendages are highly vascular ; they are about two inches in length and 

 three fourths of an inch broad, but they vary in size, having probably been largest near 

 the centre of the body, becoming smaller as they approached the sides. 



In the adult living Limulus we find the operculum, or thoracic plate, takes its rise 

 from, and is attached to, the posterior margin of the head-shield. 1 This plate bears on its 

 upper and inner surface the reproductive organs, or ovaries, and is succeeded by five 



Fig. 35. — Opercula or thoracic plates of Slimonia acuminata, Salter, sp., found associated together. 

 1. The outer or true operculum, bearing the ovaries (o). 2. First branchigerous plate underlying 1, but now displaced. 



3 ? Part of a third plate ? 



Mr. Slirnon considered this tricuspid form of plate to belong to the female of Slimonia because he has several times 



found it associated with masses of the eggs (called by Dr. Fleming Parka decipiens). 



similar but more membranous plates bearing the branchiae upon their inner surfaces. 

 These five branchial plates are nearly hidden beneath the operculum when closely 

 compressed. Each plate, however, represents the modified pair of appendages belonging 

 to a corresponding thoracic somite. 



In Plerygotus and Slimonia we find the thoracic plate, or operculum, occupying the 

 same position as in Limulus, and attached to the hinder border of the head. But as the 

 segments in the Eurypteridae are not coalesced as in the modern Limulus, but are well 

 developed, the thoracic plate can therefore only cover and conceal the two first thoracic 

 somites. 



Supposing each of these (marked viii and ix in PI. XX) to have borne a single or 



1 "With which the first thoracic segment is coalesced. This is also the case in Pterygotus, Slimonia, &c. 



