﻿PTERYGOTUS BILOBUS. 55 



Var. 1. — I. Pterygottjs bilobus, var. a, inornatus. PI. X, figs. 1 — 3. 



Himantopterus bilobus, Salter. Quart. Journ. Geol. Soc, 1856, vol. xii, p. 29, fig. 1. 



' Siluria,' 2nd edit., 1859, p. 155, foss. 21. 



'Page's Advanced Text-book,' 1856, p. 135, fig. 1. 



Ptebygotcs Salter. Mem. Geol. Surv., Mon. I, 1859, p. 39, pi. i, figs. 1—12. 



Pterygotus bilobus 1 (var. a, inornatus), the original Pt. bilobus of Mr. Salter, is 

 usually 5 to 6 inches in length, and somewhat less than 2 inches in its greatest 

 breadth ; the general form is elongate-oval in front, and attenuated behind (resembling 

 the outline of a Palceoniscus). 



In this variety the thorax is not easily distinguished from the abdomen, into which 

 it is attenuated, the greatest width being about the fourth and fifth segment ; its anterior 

 segments are wide transversely, the posterior ones becoming less and less so, till the 

 twelfth (19) is nearly equilateral ; the telson is oblong and emarginate, and narrower 

 than any of the somites ; the antennas are long, slender, and chelate at their extremities ; 

 the palpi (endognaths) are filiform ; the swimming-feet moderately broad. These are the 

 general characters. 



Pigs. 1 — 3, PI. X, represent three examples of this species, the variation observable 

 being due, to some extent, to the crumpling up, or squeezing out, which all these speci- 

 mens have undergone since their original entombment. 



*#* Before proceeding further it is necessary to explain that the little numbers along 

 the margin of the segments of each figure in the accompanying Plates correspond with 

 the Roman numerals on the segments of the restored figures of Pterygotus anglicus, PI. 

 VIII, which appeared in the Pirst Part of this Monograph. They are intended to remind 

 the student that, theoretically, the head is composed of the first seven segments of the 

 animal coalesced together to form the carapace, and that thus there are seven cephalic, 

 seven thoracic, and five or six abdominal somites or segments (according to whether the 

 'telson' be reckoned as a segment or not ; see Introduction, p. 5). 



The head-shield is about 1^ to 1^ inch in breadth by an inch or 1|- inch in 

 length ; the anterior contour is semicircular, the posterior border is straight. The com- 

 pound eyes (o, o) are placed upon the latero-anterior border. They are broadly crescen- 

 tic and convex, placed half below and half above the margin of the head, their extreme length 

 being about 7 lines. 2 No lenses can be seen with the naked eye ; but, when magnified, the 

 appearance is that given in the annexed woodcut. They appear to be somewhat larger, in 



1 In the description of the species of Pterygotus we shall (wherever it is possible) continue to avail 

 ourselves of Mr. Salter's diagnoses, merely altering such parts as a more complete acquaintance with the 

 details of their structure enables us to do so with advantage. 



s A line is always here used to signify -J^ th of an inch. 



