372 
PALAEONTOLOGY OF NEW-YORK. 
supporting a short V-shaped pit, a different muscular imprint and hinge- 
processes of the dorsal valve, and a fibrous or lamellose structure of 
shell, and are otherwise externally marked in a very characteristic 
manner. 
I have taken some pains to bring together specimens of P. (P.) elonga- 
tus, and we have now the means of knowing its real characters to a very 
considerable extent. As before observed, the dorsal valve presents all 
the characteristics of Renssel^eria in its hinge-structure and muscular 
impressions : there is the same narrow foramen beneath the hinge- 
plate, a pedicle-opening, and the filling of this is preserved in several 
specimens of the casts ; while this portion of the separated valve shows 
the same features in a most unequivocal manner, and it is quite impossi¬ 
ble to point out any characters by which it may be separated from the 
same part of the shell in Rensselasria. The crura are free from near their 
origin, and have been traced for some distance within the cavity, and 
gently curving to the ventral side ; but their termination is unknown. 
When, however, we examine the hinge of the ventral valve, we find a 
modification of the dental plates, which are prominent in Renssel.eria 
(as shown in fig. 3 g, h km, Plate cviii, Paleontology, Vol. iii), but which 
are here produced anteriorly and united at their dorsal margins. The 
incipient septum of the bottom of the valve of Renssel^eria becomes 
developed, and sustains the united bases of the dental plates; and we 
have the V-shaped pit and central septum of Pentamerus. 
In the separated valves of this species, the margins of the triangular 
fissure of the ventral valve are as clearly defined, and as free from area, 
as are the valves of Athyris or Meristella. 
Generic significations must have some limit, and we cannot consent 
to unite three so widely differing and well-marked types as P. liratus , 
P. gaspensis, etc. with P. oblongus and P. lens on the one hand, and P. elon- 
gatus on the other. 
There is also another type usually included under the Genus Pentame¬ 
rus, which may be separated with advantage. This one is represented 
by P.reversus (Billings), P.verneuili and P. interplicatus (Hall), etc. 
