SPIRIFER.E OF THE CHEMUNG GROUP. 255 



lu the Spirifers of the Carboniferous rocks which I have been able to examine, 

 this feature is variably developed. In S. plena of the Burlington limestone, the 

 septum extends for more than half the length of the fissure; while it is less de- 

 veloped in 8. grimesi, and scarcely at all in 8. increbescens and 8. opima. 



The shell texture of Spirifera is usually regarded as fibrous, and this is true 

 of all those I have examined in the older strata, though in some of the species of 

 the higher formations, characters have been observed, which, with imperfect means 

 of investigation, seem to be irregularly distributed ducts penetrating the shell. I 

 have not been able to examine the shell of 8pirif&ra alta, to learn whether any 

 changes have taken place in its texture, not observed in other species of Spirifeba* 

 but in 8. textus* where the septum and tube are developed, the shell is penetrated 

 by numerous pores or ducts which are somewhat coarser than the ordinary punctate 

 structure of some other Brachiopoda; and though not in actual contact, are often 

 arranged in regular order and frequently in close proximity to each other, not 

 essentially difiering from those wf Spiriferina as given by Dr. Carpenter. The 

 intermediate spaces have a distinctly and beautifully fil)rous or prismatic structure, 

 differing in no respect from ordinary Spirifers or Atiiyris. 



It should be observed, in this connexion, that the dental plates, both in 8piri- 

 fera alia and 8. textus, reach to the bottom of the caVity, and partially surround 

 the muscular impression, which is precisely like that of ordinary Spirifers. 



In these remarks, I have not intended to express an opinion of the generic value 

 of certain characters; but mei'cly to show, as it appears to me, a gradual or suc- 

 cessive development in certain parts, which finally becomes so wide a departure 

 from the characters of typical forms of Spirifera, as to deserve especial attention. 

 Nor can we deny that this progressive development of the septum and its mo- 

 difications keeps pace and corresponds with the geological succession; reaching its 

 extreme state, so far as now known, in the Carboniferous period, where it is con- 

 nected with a punctate texture of the shell. 



In our investigations of the Chemung group in its more easterly extension in 

 the State of New-York, it has proved, throughout the greater part of its thickness, 

 to be comparatively barren of animal organisms, though often containing an abun- 

 dance of the' remains of land vegetation. As we progress westerly, the coarser 

 sediments have given place to finer materials, or are more or less intercalated with 

 shales or shaly and calcareous beds, while the coarser beds exhibit less resemblance 



to shore deposits, and we find an increasing number of animal remains both of the 

 » , ___ 



• In this species, and in Spiriferina spinosa of Norwood &, Pratten, the pores or ducts are un- 

 equally distiibuted ; being, in some parts of the shell, closely crowded, while in others they are more 

 distant and often following the line of growth, though frequently irregularly disposed. 



Prof Wi.vciiELL remarks, of the proposed Genus Stbingothtris, that "the shell is impunctate 

 in all conditions and under high powers." 



