PTERYGOTUS BILOBUS. 55 
Var. 1.—I. Prerycotus BILosus, var. a, inornatus. Pl. 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. 
a —— ‘Page’s Advanced Text-book,’ 1856, p. 135, fig. 1. 
PTERYGOTUS —— Salter. Mem. Geol. Surv., Mon. I, 1859, p. 39, pl. i, figs. 1—12. 
Pterygotus bilobus* (var. a, inornatus), the original Pt. dclobus 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 Pal@oniscus). 
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 antenne are long, slender, and chelate at their extremities ; 
the palpi (endognaths) are filiform ; the swimming-feet moderately broad. These are the 
general characters. 
Figs. 1—3, Pl. 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, P|. 
VIII, which appeared in the First 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 14 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 (0, 0) 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.” 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. 
2 A line is always here used to signify ;';th of an inch. 
