from the Coal-measures of Illinois. 21 
rior margin of the fifth thoracic segment, or to a length of 1-60 
inches, while we are almost positively sure it is not bipartite 
at the extremity. There is also on each side of the anterior, 
or attached extremity of the mesial appendage, a small spatu- 
late piece, not corresponding to any of the parts of the opercu- 
lum of Furypterus, as hitherto illustrated, so far as we have 
yet seen. ‘These are about 0°41 inch in length, and 0°15 inch 
in breadth, with nearly parallel sides and pointed anterior ex- 
tremities, while their posterior ends are transversely truncated, 
with lateral angles rounded. Their anterior pointed ends ter- 
minate nearly in contact with the posterior angles of the two 
little “intercalated pieces” (a a) of Prof. Hall’s fig. 3, p. 398, 
of third vol. Paleont. N. Y.; so that they occupy exactly the po- 
sition of what are apparently intended in that figure to represent 
the inner truncated ends of the lateral alee, immediately on each 
side of the mesial appendage. They are proportionally wider, 
however, and extend back slightly beyond the posterior mar- 
gins of the lateral alee, and are certainly separate pieces. They 
were evidently over-lapped, on their mner edges at least, by 
the mesial appendage, and look as if, in case they were attach 
at all to the operculum, it must have been to its inner side. 
Possibly, however, they were really attached to the anterior tho- 
racic segment, and form no part of the operculum itself; in whic 
case they would seem to represent, though greatly smaller, the 
membranaceous modified feet of Limulus, bearing the branch- 
ie. As now seen, however, in the condition of impressions, 
they certainly look as if appendages of the operculum; while 
they show the same scaly sculpturing seen on other parts of 
the surface. 
All the portions of the under side of the fossil, that have 
left their impressions in the matrix, were provided with fine 
subimbricating scale-like markings. 
From some of the characters mentioned, particularly the 
in having no eyes, at least in the position they oceupy in Bw- 
ng no eyes, Shs posite ey i 
