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, Avhere 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 Helderberg, <8. consohrinus* of the Hamilton group and 8. 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 Spirifer- 
INA, and, indeed, form the only basis of distinction between some of the palaeozoic 
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 described as Spirifer zic-zac, Hall, in 1843. The same specific name was, curiously 
enough, used by F. Roemer, in the same year, for a quite distinct Devonian Spirifer, and D’Orbigny, in 
1850, proposed for the Ameidcan species the name above used. 
fSee observations on the genera Syringothyris, Cyrtina and Spiriferina. 
