54 BRITISH FOSSIL CRUSTACEA. 
Genus 1.—Preryeorus, Agassiz.— Continued. 
Species 2.—PTERYGOTUS BILOBUS :—Salter. 1859. 
1. var. a, zornatus. 3. var. y, perornatus. 
2. var. B, crassus. 4. var. 6, acidens. 
The species about to be considered was first described by Mr. Salter, under the 
generic name /imantopterus, in the ‘Quarterly Journal of the Geological Society’ for 1855, 
vol. xu, pp. 27 and 28, where a figure is also given (reproduced in ‘ Siluria,’ last edition, 
S67, p..162): 
Of the species of Himantopterus, described in the paper above alluded to, two only at 
present call for consideration, namely, 7. dc/obus and H. perornatus. My. Salter subse- 
quently referred these to Pterygotus (see ‘Mem. Geol. Surv.,’ Monograph 1, 1859, p. 39). 
At the time of the publication of the above Monograph comparatively but few entire 
remains of Pterygotus bilobus had been obtained, whilst P¢. perornatus was even less 
perfectly known from parts of specimens. 
It is probable that Mr. Slimon has obtained not fewer than several hundred specimens, 
more or less perfect, of P¢. di/obus alone, since the commencement of his labours in the 
shales of Lesmahago, but P¢. perornatus continues an extremely rare form. 
If a comparison be instituted between the figures in the first plate of the Geo- 
logical Survey Monograph on Péerygotus (published in 1859) and the six plates which 
accompany the present part of this Monograph, some idea may be formed of the progress 
which has been made in the discovery of new materials, during the past nine years, for the 
more complete elucidation of this remarkable genus.” 
1 This progress will be still further illustrated by the genera Slimonia, Eurypterus, and Stylonurus, 
the descriptions of which will follow the present part. 
