252 TWENTIETH REPORT ON THE STATE CABINET. 



rated, and, extending inwards, are recurved, their edges sometimes 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 S. textus, or an 

 incipient step towards it, in 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 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 open- 

 ing, but apparently not connected with, nor a part of the dental lamellas 

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

 extension of the 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 Spirifera textus ; which, so far as I can now deter- 

 mine, is identical with the one described by me as <S. suhcuspidatus in the 

 Geological Report of Iowa. The latter is a carboniferous species from the 

 Keokuk limestone, and closely allied to the 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 regarded 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 thicken- 

 ing 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 7nacropleura hsiB the 



