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 (6) 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 

 nari'ower 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.! With P. tenuistriatus may be associated the American species 



* Geologie de la Russie et des Mont, de I'Dural, p. 111. 



t Nettblroth has desci-ibed as P. Knighti a shell from the Coi-niferous limestone near Louisville, Ky. 

 (Kentucky Fossil Shells, p. 57, pi. xxix, tigs. 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. Pentamtrus Littoni, 

 Hall, of the Niagara group, is another representative of the P. Knighti type of exterior. 



