GENERA PENTAMERUS, &o. 
371 
continued as far as the middle of the length of the valve, where their 
extremities apparently become free. In these features, there is no essen¬ 
tial difference between P. oblongus and P. knightii or P. galeatus. 
In Pentamerus lens, or a closely allied form, from Anticosti, the dorsal 
valve possesses similar characters, and there is no area on the ventral 
valve. Restricting the designation to such forms as P. liratus and P. micro- 
carrier us among the European species, there is good ground for the sepa¬ 
ration ; but the other species do not appear to me congeneric. Through 
the kindness of Sir W. E. Logan, I have been permitted to examine 
the American species of Stricklandinia, S. gaspensis , S. canadensis and 
S. anticostensis ( Billings), and also the species referred to the European 
P. lens. The first two appear to me to be congeneric with P. liratus, and 
do possess the characters of the genus as described. Limiting, therefore, 
the application of the term as here indicated, I think we have a well- 
marked genus, which, typified by the species above named, may also 
include others heretofore referred with doubt to Spirifera. I conceive, 
however, that our appreciation of generic limitations will not be 
enhanced by including under the same term the P. lens and P. oblongus 
= P. Icevis. 
Though at first restricting the genus to the Middle Silurian forma¬ 
tions, Mr. Billings has, at a later period, included under Strickladinia 
the Pentamerus' elongatus of Vanuxem = Rensselceria elongata (Ball), a 
Devonian species. This form was first placed und'er Pentamerus, from its 
resemblance to P. oblongus ; and I finally referred it to RensselcEria, 
from the generally similar shape, similar muscular impressions of 
the dorsal valve, and the prismatic or punctate texture of the shell; 
having at that time an imperfect knowledge of the interior structure of 
the ventral valve. Now this species has no area on the ventral, valve; 
but it has a V - shaped pit supported by a septum precisely as in Penta¬ 
merus ; while both the muscular impression of the dorsal valve, and the 
structure of the hinge-plate, are precisely the same as in Rensselceria. 
I cannot see, therefore, upon what good ground this shell should be asso¬ 
ciated with P. liratus, P. gaspensis, P. canadensis and P. anticostensis , which 
have a straight hinge-line, an area on the ventral valve with a septum 
