«5« PALEONTOLOGY OF NEW- YORK. 



higher position. The high area and the transverse concave septum, which is not a 

 true pseudo-deltidium, allies it with Spirifera textus, in which we find similar fea- 

 tures. In the S. alia 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 lamellae, 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 Spirifera 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 thero 

 is a tubular callosity; or, in other words, the inner lamina) 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 laminie 

 appear to be separated, and, extending inwards, are recurved, their edges some- 

 times joining to fonn 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 S. textus, or an incipient step 

 towards \i,'va. Spirifera 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 ha.s 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 tho 

 opening, but apparently not connected with, nor a part of the dental laraclloe or 

 substance of the area, while the septum is an outgrowth or lateral extension of tho 

 dental plates. 



In the case of Spirifera 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 

 Spirifei-a textus ; which, so far as I can now determine, is identical with the one 

 described by me as S. sufjcuspidatus in the Geological Report of Iowa. The latter 

 is a carboniferous species from tho Keokuk limestone, and closely allied to the 



• It 18 upon this feature, or one of similar character, in its full developnient, that I understand Prof. 

 WiNcaELL proposes to found, the Genus Strimgotbtbis. 



