No. 50.] 
205 
two. No portion of the original crust remains. It may thus be cha- 
racterized ; ribs numerous, narrow, prominent, with a longitudinal groove 
tuberculated on the margins ; ribs of the middle lobe somewhat angu- 
lated in the middle, the angle pointing downwards ; margin of the tail 
regularly rounded, with a broad flat depression between it and the ribs, 
finely striated. A buckler was found in the same locality, which is 
probably of this species. It has a general resemblance to the buckler 
of A. longicaudatus of Murchison, but the oculiferous tubercles are far 
larger. 
Acidapsis tuherciilatus . 
A very small species much like a Limulus, especially in the tail which 
is a long spine ; the angles of the buckler are elongated ; the whole sur- 
face of the buckler covered with unequal pustules ; on each side of the 
middle lobe are two short elliptical tubercles. It is very dissimilar to 
the species described by Murchison, and much smaller. I found one 
specimen in the shaly limestone or Wenlock equivalent, near Clarks- 
ville, Albany county, in the bank of a stream. 
Acanthaloma. 
This is a fragment apparently of the buckler of a most singular spe- 
cies; it is elongated into a curved spine and has a row of spines along 
the front, and three spines on the opposite side of the prolongation, 
not uncommon in the shaly limestone near Clarksville. 
Univalve Shells. 
Platyceras. 
I propose to group in this genus the Pileopsis tubifer, Sowerhy^ P. ve- 
tusta, Sowerhy, the Nerita haleotes, Sowerhy, and perhaps Bellerophon 
cornu-arietis. The shells are suboval or subglobose, with a small spire, 
the whorls of which are sometimes free and sometimes contiguous ; the 
mouth is generally campanulate or expanded. I have seen a species 
above the Silurian rocks, though they probably occur above them in 
Europe, and they are never found in the Lower Silurian strata, They 
characterize the middle portion of the system. 
Platyceras diimosum. 
Shell covered with thick tubular spines arranged in longitudinal rows ; 
margin of aperture waved ; volutions free. 
