16 PALEONTOLOGY OF NEW YORK. 



bear a low median depression on the fold accompanied by a corresponding 

 median ridge in the sinus. 



In this division we meet with considerable diversity in exterior form, the 

 hinge being at times short, and again extremely extended ; always, however, 

 making the greatest diameter of the shell. The cardinal area is usually low, 

 but may be considerably elevated. The species all have a fine, very faintly 

 developed median ridge in the interior of the brachial valve and the cardinal 

 process developed as a low, single, multistriate apophysis, with sometimes a 

 tendency to bilobation. 



The lamellose species are conveniently subdivided into two groups : 



1. Septati; those having a median septum in the pedicle-valve. The septum 

 lies between the bases of the teeth but does not come into contact with them 

 as in the genus Cyrtina, where the latter are supported by dental lamellae rest- 

 ing on the bottom of the valve. 



This character is found in an incipient condition of development in the 

 Niagara species Spirifer sulcatus, Hisinger, and is a more conspicuous feature in 

 subsequent forms, S. perlamellosus, of the Lower Helderberg, S. raricosta, of the 

 Upper Helderbex'g, S. consohrinus* of the Hamilton group and S. mesacostalis, of 

 the Chemung group. Up to the period of the upper Devonian, at least in Amer- 

 ican faunas, the existence of this septum in the pedicle-valve is not accompanied 

 by a punctation of the shell-tissue, nor by the union of the processes on the pri- 

 mary lamellae of the spiral arms; features which characterize the genus Spirifek- 

 INA, and, indeed, form the only basis of distinction between some of the paljBOzoic 

 members of this genus and these septate Spirifers. At present we are without 

 evidence of the gradual assumption of punctation by shells in this line of 

 development, but there can be no reason to doubt that its appearance here was 

 of the same nature as along the line leading from Spirifer to Syringothyris,! 

 gradual or sporadic. 



* This is the species desci-ibed as Spirifer zie-zac, Hall, in 1843. The same specific name was, curiously 

 enough, used by F. Roemer, in the same year, for a quite distinct Devonian Spiripeb, and D'Orbignt, in 

 1850, proposed for the Ameiican species the name above used. 



t See observations on the genera Syringothtri.s, Cyrtina and Spiripbrina. 



