SPIEITER^ OF THE CHEMUNG GROUP. 345 



The interior of the ventral valve shows the presence of more or less 

 extended dental lamellse : these are sometimes short and divercrent. 

 enclosing a pretty broad muscular space, and often less divergent and 

 reaching nearly to the front of the shell ; and the muscular imprint is 

 sometimes pushed very far forward, and enclosed in a narrow space. 

 The muscular markings are often well defined in the casts, but in some 

 of them the markings of the ribs are shown over the muscular space. 

 The fissure is closed for one-third or more of its length from the apex by 

 a thickened plate or septum, with the rostral cavity more or less ex- 

 tended behind it. In casts of the dorsal valve, we have the imprints of 

 the cardinal muscles and the cavities made by the bases of the crura. 



This species has been recognized by European authors as Spirifera 

 disjunda, and identical with S. verneuili, etc., as shown in the synonymy. 

 From comparison of ours with European specimens of that species in my 

 own collection, I can find no important distinction ; indeed the differences 

 in a few indviduals are not as great as those among our own Specimens 

 recognized as belonging to the same species. It is regarded in Europe as 

 a very characteristic Middle and Upper Devonian species; but in the 

 State of New-York, it is not known out of the Chemung group. 



The Spirifera whitneyi is from beds of the age of the Hamilton group 

 in Iowa, and bears a very close resemblance to the S. archiaci as figured 

 in the Geology of Russia (ut cit). Some specimens of this form more 

 recently examined* show a variation in aspect and general characters 

 which induce the conclusion that this one also is only a variety of S. 

 disjunda, occurring at a lower horizon in the west than yet known in 

 New York ; thus giving it a vertical range equal to the Middle and Upper 

 Devonian of Europe. 



I have heretofore! given some of the varieties of this species under 

 distinctive names, which have been abandoned after the study of larger 

 collections. I have also been able to satisfy myself with regard to the 



• Specimens received from Mr. 0. St. John, of Waterloo, Iowa, 

 t Geological Report of the Fourth District of New- York, pp. 270, 271. 



