128 
PALAEONTOLOGY OF NEW YORK. 
the Final Report of the United States Geological Survey of Nebraska, and re¬ 
produced upon Plate IV f, fig. 31. 
It is now easy to find an explanation of the apparently great variation in the 
length of the groove in these fossils. Even in a given species it will appear 
sometimes short, at other times seeming to reach nearly or quite to the margin, 
according as the specimen shows the outer or the inner surface. Unquestion¬ 
ably the length of the external groove is a matter of specific variation to a 
slight extent only, its development keeping pace with the age of the shell, 
while the length of the internal furrow appears to be, to a much more consider¬ 
able degree, susceptible of variation in a given species. 
Leaving the further discussion of the characters of these fossils for subse¬ 
quent reference, we may turn to the consideration of the generic term: 
Orbiculoidea, D’Orbigny, 1847. Considerations zoologiques et geologiques 
sur les Brachiopodes; Comptes rendus, vol. xxv, p. 269. 
The definition of this term was first given in the Prodrome de paleon- 
tologie stratigraphique, 1849, and is in the following terms: “ Coquille de 
contexture cornee non perforee, dont la valve inferieure concave est pourvue 
d’un ouverture laterale ou crochet pour le passage d’un pedicule simple,” the first 
example cited under this definition being the Orbicula Morrisi, Davidson. Mr. 
Dall has observed* that in neither the first use of the term,f nor in the sec¬ 
ond,! was an example cited, and, therefore, reaches the unavoidable conclusion 
that 0. Morrisi, being the first citation made by the author, must be assumed 
as the typical species. Mr. Davidson, in 1853,§ was disposed to consider 
Kutroga’s genus, Schizotreta, as synonymous with Orbiculoidea, and took 
Kutorga’s type, S. elliptica, as the type of the latter genus. Subsequently 
(1866),|| holding to the equivalence of these terms, he did not modify his 
opinion as to the type-species of Orbiculoidea. 
* Bulletin No. 8, U. S. National Museum, p. 51. 1877. 
t Comptes rendus, vol. xxy, p. 269. 
t Ann. Sci. Nat., vol. xxx, p. 351. 1850. 
• § Introd. British Fossil Brachiopoda, pp. 129-131. 
|| Silurian Brachiopoda, pp. 72, 73. 
w\»Vt \it. 
