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 Pl. IX, figs. 1 
and 1c). I do not think them of specific importance. 
Some excellent figures of Pterygotus bilobus will be found in pl. 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 Pt. dilodus, var. a 
(figs. 1—12). In pl. 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 antenne 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.’ (See Part I of this Monograph, Pl. VIII, fig. 1, &.) 
In pl. 1, figs."8 and 8 a (op. evt.), a very perfect swimming-foot of one of the ectognaths 
is represented having a long stylet (¢) attached to the lower and inner border of the 
Ath 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, Hurypterus lanceolatus) 
belong to Pterygotus bilobus (var. y, perornatus), to be presently described. 
Var. 2.—PrERYGOTUS BILOBUS, var. 3, crassus. Pl. 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. 1855, 
‘Quart. Journ. Geol. Soc.,’ 1856, vol. xii, p. 28 ; and in ‘ Siluria,’ 4th edition, 1867, p. 162. 
