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- 

 al)ly 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 grologiques 

 sur les Brachiopodes ; Comptes rendus, vol. xxv, p. 269. 



The definition of this term was first given in the Prodrome de palron- 

 tologie stratigraphique, 1849, and is in the following terms: " Coquille de 

 contrxture corm'-e non perfon'e, 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. 

 Dai.i. has observed* that in neither the first use of the term,f nor in the sec- 

 ond,J: was an example cited, and, therefore, reaches the unavoidable conclusion 

 tliat 0. Morrisi, being the first citation made by the author, must be assumed 

 as the t\-pical species. Mr. Davidson, in lSo3,§ was disposed to consider 

 Kutroga's genus, Schizotreta, as synonymous with Orbiculoidea, and took 

 Kutorga's type, S. ellipiica, as the type of the latter genus. Subsequently 

 (1866), II holding to the equivalence of these terms, he did not modify his 

 opinion as to the type-species of Orbiculoidea. 



* Bulletin No. S, U. S. National Museum, ji. 51. 1877. 



t Comptes rendus, vol. xxv, p. 269. 



J Ann. Sci. Nat., vol. xxx. p. 351. 1850. 



§ Introd. Bi-itish Fossil Bnichiopoda, jip. 12!l-131. 



I Silurian Brachiojioila, i)p. 72, 73. 



