292 REPORT ON THE STATE CABINET. 



scai'cely to merit the term septum. In Spirifera marcyi this development of 

 shelly matter has the character of a true septum, closing one-third or more 

 of the length of the tissure from above, and leaving an open rostral cavity 

 behind it. In S. medialis and 8. macronota there is a thickened transverse 

 septum extending from one-third to one-half the length of the fissure from the 

 apex, and this feature is quite independent of the pseudo-deltidinm. The rostral 

 cavity lies behind this septum, and may be open or free from the accumulation 

 of shelly matter to a greater or less extent; but it is not uncommon to find 

 nothing more than a thickening of the base of the dental plates and a partial 

 filling of the rostral cavity. This condition changes to a greater or less extent 

 during the growth of the shell; but a partial filling of the fissure at its apex 

 is probably more common than otherwise, particularly in full-grown individuals. 



"In the Spirifers of the Carboniferous rocks which I have been able to 

 examine, this feature is variably developed. In 8- plena of the Burlington 

 limestone, the septum extends for more than half the length of the fissure; 

 while it is less developed in 8. grimesi, and scarcely at all in 8. increhescens 

 and *S'. opima. 



"The shell texture of Spieifera 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 formation, 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 8pirifera 

 alia, to learn whether any changes have taken place in its texture, not observed 

 in other species of Spieifera; 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 Brachio- 

 poda; and though not in actual contact, are often arranged in regular order 

 and frequently in close proximity to each other, not essentially ditl'ering from 

 those of Spiriferina as given by Dr. Carpenter. The intermediate spaces 

 have a distinctly and beautifully fibrous or prismatic structure, differing in no 

 respect from ordinary Spirifers or Athyris. 



" It should be observed, in this connection, that the dental plates, both iu 8pin- 

 fera alta and 8. textus, reach to the bottom of the cavity, and partially surround 

 the muscular impression, which is pi'ecisely like that of ordinary Spirifers. 



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

 value of certain characters; but merely to show, as it appears to me, a gradual 

 or successive development in cei'tain parts, which finally becomes so wide a 

 departure from the characters of typical forms of Spieifera as to deserve 

 especial attention. Nor can we deny that this progressive development of the 

 septum and its modifications keeps pace, and corresponds with the geological 

 succession; reaching its extreme state, so far as now known, iu the Carboniferous 

 period, where it is connected with a punctate texture of the shell."! 



* In this species, and in Spiriferina spinosa of Norwood and Pbatten, the pores or ducts are 

 unequally distributed ; 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. WiNCHELL remarks of the proposed Genus Sykingothtbis, that " the shell is impunc- 

 tate in all conditions and under high powers." 



t See note on page 256. 



