﻿62 BRITISH FOSSIL CRUSTACEA. 



Probably these differences in form may be of sexual value, especially as the same 

 portion of this plate is modified in the male and female of Limulus (see PI. IX, figs. 1 

 and \ c). I do not think them of specific importance. 



Some excellent figures of Pteryyotus bilobus will be found in pi. i, ' Memoirs of the 

 Geol. Surv./ Monograph I, 1859. In this plate are represented two entire examples, 

 parts of three others, and several detached appendages belonging to PL bilobus, var. a 

 (figs. 1 — 12). In pi. xv of the same work a diagrammatical restoration is given of 

 this species, which, however, now needs to be corrected in accordance with the present 

 more complete knowledge of its structure. Another segment should be added to the 

 thoracic series, and between the chelate antennas and the great ectognaths or maxillipeds, 

 there should be three pairs of endognaths with simple palpi. 



The true position of the thoracic plate or operculum has also since been satisfactorily 

 made out. 1 (See Part I of this Monograph, PI. VIII, fig. 1, &c.) 



In pi. i, figs. *8 and 8 a {pp. cit.), a very perfect swimming-foot of one of the ectognaths 

 is represented having a long stylet (t) attached to the lower and inner border of the 

 4th joint (meros). This is evidently the palpus of one of the endognaths detached from 

 its place and lying upon the surface of the swimming-foot of the ectognath, a very likely 

 thing to happen when the appendages become detached from the head. 



The other remains figured in this plate (except fig. 17, Eurypterus lanceolatus) 

 belong to Pteryyotus bilobus (var. y, perornatus), to be presently described. 



Var. 2. — Pterygotus bilobus, var. |3, crassus. PI. XI, fig. 1. 



The specimen upon which this variety is founded is distinguished from var. a by 

 the greater transverse breadth of the body-segments in proportion to their length, whilst 

 the penultimate segment (19) and the telson (20) are as long and as well developed as in 

 the preceding variety of this species. 



Nor does it appear that any of the segments save the last thoracic (14) and the first 

 abdominal (15) have been abnormally overlapped one above another by longitudinal 

 compression after death. 



1 These same corrections may be made in the first figure ever given by Mr. Salter, in Nov. 1355, 

 ' Quart. Journ. Geol. Soc.,' 1856, vol. xii, p. 28 ; and in ' Siluria,' 4th edition, 1867, p. 162. 



