SPIRIFERiE OF THE CHEMUNG GROUP. 
253 
Spirifera cuspidatus of Europe; and the Chemung species thus reminds us of the 
Carboniferous one, or has a carboniferous aspect. 
If, in its full development, the presence of a septum and internal tube be re¬ 
garded as of generic value, then we have in Spirifera alta the same appendages 
in part, or in a partially developed condition; the distinct tube only being wanting. 
But had we the means of examining the internal characters of the ventral valves 
of all the species of Spirifers, we should probably find gradations from the solid 
filling of the rostral cavity, with a greater or less extension of shelly matter in the 
form of a septum in the fissure occupying a narrow space in its apex, till we reach 
the development observed in S. alta and S. textus. 
To a considerable extent, we are able to prove this supposition. In the separated 
valves of Spirifera radiata of the Clinton and Niagara groups, there is a filling of 
the rostral cavity, and a thickening of the dental lamellae which are extended for 
half the length of the valve. The narrow median crest of the muscular impression 
is expanded and thickened towards the apex of the valve, but there is no evidence 
of a septum. In Spirifera niagarensis there is a filling of the rostral cavity and a 
thickening of the dental lamellae, which are distinctly separate, but no appearance 
of a septum; and there is a low median crest in the muscular impression. 
In the Lower Helderberg group, the Spirifera macropleura has the rostral 
cavity filled to a greater or less extent, and the dental plates extremely thickened 
at their bases; while there is an obscure low median crest in the muscular impres¬ 
sion, which sometimes becomes thickened and expanded above; and occasionally 
there is a little filling of the upper part of the fissure, resembling the incipient 
stages of growth of the septum. In Spirifera cycloptera the rostral cavity is more 
or less filled and solid, with diverging dental lamellae; while the crest dividing 
the muscular impression is very unequally developed, and there is no appearance 
of a septum. In Spirifera perlamellosa the rostral cavity is filled to a greater or 
less degree, the dental lamellae are short and strong, and there is a sharp elevated 
median crest or septum in the muscular impression; but ( there is no indication of 
the transverse septum. In Spirifera octocostata the dental lamellae are thickened, 
and there is a vertical septum reaching from the apex of the rostral cavity to the 
base of the muscular impression. In Spirifera marylandica of the Oriskany sand¬ 
stone, there is sometimes a partial filling of the rostral cavity, with a reversed 
conical crest in the upper part of the muscular area. Spirifera arenosa has strong 
dental lamellae; a filling of the rostral cavity in old shells; a low crest dividing 
the muscular impression, which becomes thickened in its upper part; a distinct 
pseudo-deltidial covering of the fissure, but no transverse septum. 
Among the Spirifers of the Upper Helderberg group, Spirifera acuminata has 
usually only a moderate thickening of the rostral portions, divergent lamellae of 
