DESCRIPTION OF THE SPECIES OF PTERYGOTUS 205 
PLaTeE I, [PLATE 12] Fics, 1-12. 
SECTION 1. 
PTERYGOTUS BILoBUs. 
P. 8-uncialis, subleevis, capite semielliptico, corpore abbreviato antice latiort, 
caudd oblongé carinaté emarginatd. 
Synonym. Aiimantopterus bilobus, SALTER, in Quart. Journ. Geol. Soc. 
vol. xil., p. 29, fig. « ; also “Siluria,” 2nd edition, p. 155, foss. 21. Par, in 
Advanced Textbook, p. 135, fig. r. 
We have more perfect materials for this species than for any of 
the rest,—above twenty specimens, many of them full grown, and 
showing all the segments ; in some the head with its elongate eyes 
and few-jointed antenne; the long palpi in others; two show the 
serrated jaw feet beneath the carapace with the swimming joints 
attached ; the post-oral plate is cz st in another; and there are 
numerous detached specimens of all the appendages except the palpi, 
of which last we should know but little but for the explanation 
afforded by the next described species. 
The geological position of this and the other Prerygot? from 
Lesmahago has been fully given by Sir R. I. Murchison, our Director- 
General, in the 12th volume of the Quarterly Journal of the Geological 
Society, p. 23. The occurrence with them of a small spiral shell and 
a Lingula, identical with those of the Downton Sandstones of 
Shropshire, justifies the reference of the strata to the age of the 
Uppermost Ludlow Rocks, while not one of the species is identical 
with those of the border counties of Wales, either in the Ludlow 
Rocks or in the Passage Beds which immediately overlie them. 
This fact is remarkable, and may serve to indicate the probability 
of a vast number of species yet to be discovered. 
Description—The general form of H. dzlobus is elongate-oval in 
front and attenuated behind (resembling a good deal the outline of 
a Pal@oniscus or other fish). The thorax is not easily distinguished 
from the abdomen, into which it is attenuated, the greatest width 
being about the third or fourth segment; its segments are widely 
transverse, those of the abdomen become less and less so, till the last 
but one is nearly square; the tail joint is oblong and emarginate, 
and narrowest of all ; the antenna are slender and long, the swimming 
feet narrow, the palpi filiform; and these general characters, taken 
together with the small size (seldom nine inches long), will easily 
