234 
PALEONTOLOGY OF NEW YORK. 
pact as it may seem in both external and internal structural features, has 
apparently been developed along various lines from a central origin, and when 
such variations are considered in connection with certain established claims of 
nomenclature, a further subdivision of these shells will prove both useful and 
requisite. 
The essential foundation for a subordinate grouping of these pentameroids 
was indicated by de Verneuil so long ago as 1845,* when he proposed a divi¬ 
sion into two sections; (1) those without a sinus, (2) those with a sinus. The 
former of these was divided into {a) plicated shells, including P. Knighti, P. bilo- 
culare, etc.; and (Jb) smooth shells, such as P. oblongus, P. borealis, etc. The 
second section included shells of the type of P". galeatus. These three divisions 
indicate the main lines of variation in external characters. 
To the plicated species without well defined fold and sinus must be applied 
Linne’s original term Conchidium, founded in 1753 upon the Swedish species, 
widely known as Gypidia conchidium, Dalman, which is identical with Conchidium 
biloculare, Linne. The diagnosis of this genus, above given, has been derived 
from an abundant representation of specimens of the species, and, in respect to 
some critical features, with the aid of the elaborate illustration given in 
Angelin’s (Lindstrom’s) “ Fragmenta Silurica.” This shell is peculiar in its 
elevated and unciform beak; in this as well as other respects it is homologous 
with the much larger and more robust American shell, P. laqueatus, Conrad (= P. 
nobilis, Emmons), which occurs in enormous quantities in the Niagara dolomites, 
about Delphi, Indiana. Usually these plicated species have a lower, though 
narrower beak, and are constructed on the plan of P. tenuistriatus, Walmstedt, 
of the Upper Silurian of Gotland; those with the broader form and almost 
subquadrate sectional outline, like the well-known P. Knighti, Sowerby, being of 
rare occurrence.f With P. tenuistriatus may be associated the American species 
* Geologie de la Russie et des Mont, de I’Oural, p. 111. 
t Nbttelroth has described as P. Knighti a shell from the Corniferous limestone near Louisville, Ky. 
(Kentucky Fossil Shells, p. 57, pi. xxix, figs. 12, 17). While the species is similar in general contour to the 
English Silurian shell, it is much smaller and more coarsely plicated, and it must be regarded as a quite 
distinct form, which for convenience’s sake may be known as Conchidium Nettelrothi. Fentamerus Littoni, 
Hall, of the Niagara group, is another representative of the P. Knighti type of exterior. 
