PALAEONTOLOGY OP NEW-YOKK. 
higher position. The high area and the transverse concave septum, which is not a 
true pseudo-deltidium, allies it with Sjpirifera textus, in which we find similar fea¬ 
tures. In the 8 . alta there has probably been an external convex pseudo-deltidium, 
and between this and the septum closing the fissure there has been a narrow space. 
This septum, which is an extension of the dental lamellse, has been thickened or 
expanded on the inner side, as shown by the casts of the ventral valve; and in 
several specimens there is a narrow semicylindrical depression extending nearly to 
the beak of the valve. 
In comparing this species with Sjpirifera textus, we find similar conditions, or 
more properly an extension or amplification of the same features. In that species 
there is a convex arching pseudo-deltidium, though rarely preserved in the speci¬ 
mens. Beneath this there is a concave septum, and upon the inner face of this there 
is a tubular callosity; or, in other words, the inner laminte of the septum become 
fistulous, and enclose a cylindrical or subcylindrical space, which extends from the 
base of the septum to near the apex of the valve*. But more usually the laminae 
appear to be separated, and, extending inwards, are recurved, their edges some¬ 
times joining to form a tube; but more frequently perhaps the margin of each one 
is recurved upon itself, leaving the tube with a slit along the lower side. In some 
instances, however, these extensions from the inner face of the septum continue to 
the bottom of the cavity, and, joining the external shell, leave a quadrangular tube 
instead of a cylindrical one. 
It is evident from what has been said, as well as from the illustrations given, 
that there is a near approach to this character of 8. textus, or an incipient step 
towards it, in. Sjpirifera alta ; but the feature has not become fully developed. We 
have the concave septum with a semicylindrical callosity on its inner face, but this 
ridge has apparently remained solid. The concave septum, however, must not be 
confounded with the pseudo-deltidium : the latter is apparently an independent 
growth of shelly (or sometimes perhaps scarcely solid shell) matter, forming an 
exterior plate from the apex of the fissure, covering to a greater or less extent the 
opening, but apparently not connected with, nor a part of the dental lamellse or 
substance of the area, while the septum is an outgrowth or lateral extension of the 
dental plates. 
In the case of Sjpirifera alta, the extension of the septum is so great, that when 
connected with the general form of the shell, its nearest analogue appears to be 
Sjpirifera textus ; which, so far as I can now determine, is identical with the one 
described by me as 8. subcusjpidatus in the Geological Eeport of Iowa. The latter 
is a carboniferous species from the Keokuk limestone, and closely allied to the 
* It is upon this feature, or one of similar character, in its full development, that I understand Prof. 
Winchkll proposes to found the Genus Syringothyris. 
