37© 
PALAEONTOLOGY OF NEW-YORK. 
“ beneath the beak as in Pentamerus. In the dorsal valve, no longitudinal sep- 
“ turn, spires or loop ; the whole of the internal solid organs consisting of two 
“ very short or rudimentary dental plates, which, in some species,, bear pro- 
“ longed calcified processes for the support of the cirrated arms. In all the 
“ species, the ventral valve has an area more or less developed. 
“ This genus includes three English species, which have been loug known under 
“ the names of Pentamerus lens , P. liratus and P. Icevis. All these, and the 
“ three Canadian species, abound in rocks of the age of the Middle Silurian, 
“ such as the Llandovery rocks of Sir R. Murchison, and the Clinton and 
“ Niagara groups of the New-York geologists. No species have as yet been 
“ found either above or below the Middle Silurian. On the other hand, the 
“ Genus Pentamerus occurs more or less frequently in all formations from the 
“ Black-river limestone to the Devonian inclusive.” 
The European Pentamerus liratus undoubtedly belongs to a genus dis¬ 
tinct from P. knightii, having a straight hinge-line, an area on the ven¬ 
tral valve, with a sensible mesial depression and corresponding eleva¬ 
tion on the opposite valve. The species has likewise a short septum, and 
supports a small triangular pit in the ventral valve; while in the dorsal 
valve the hinge-plate is divided, and the parts are extended in long 
lamellae into the interior of the valve. The Pentamerus microcamerus, 
M‘Coy (= Spirifera? Icevis, Sowerby), belongs also to the same group, 
having a straight hinge and narrow area, with a short V - shaped pit; 
characters unlike those of true Pentamerus. At the same time, the Pen¬ 
tamerus lens and P. Icevis, which are, by Mr. Billings, united with 
P. liratus under the Genus Stricklandinia, appear to me to differ very 
widely from that species. They have no area, in the proper acceptation 
of that term, and the hinge-line is not extended. In the ventral valve, 
the long V-shaped pit is supported on a septum which sometimes extends 
for nearly half the length of the valve. Moreover, the dorsal valve in 
the American species of P. oblongus is marked by the presence of very 
extended lamellae, which are united at their origin and spread laterally 
till their free margins meet .the corresponding margins of the lamellae 
forming the V-shaped pit in the ventral valve; and running parallel 
with it for nearly its entire length they then become vertical, and are 
