254 TWENTIETH REPORT ON THE STATE CABINET. 



attenuated towards the centre of the muscular impression. This callosity 

 sometimes becomes so prominent as to produce an emargination or indenta- 

 tion in the apex of the cast, and a similar feature is sometimes observed 

 in the casts of other species of Spirifera. 



"Regarding the S. granulifera alone, there is little to attract especial 

 notice, beyond the general fact of a partial filling of the rostral cavity with 

 the exterior portion prolonged between the dental lamellae, but so much- 

 thickened as scarcely 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 fissure from above, and leaving an 

 open rostral cavity behind it. In Spirifera medialis and S. 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-deltidium. The rostral cavity lies behind this septum, and 

 may be open or free from the accumulation of shelly matter to a greater 

 or leas extent ; but it is not uncommon to find nothing more than a thicken- 

 ing 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 Spirifcrs 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 developed in S. grimesi^ and scarcel}'- at all in S. increhes- 

 cens and <S. 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 

 Spirifera alta, to learn whether any changes have taken place in its tex- 

 ture, not observed in other species of Spirifera ; but in S. 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 differing from those of Spiriferina as given 

 by Dr. Carpenter. The intermediate spaces have a distinctly and beauti- 

 fully fibrous or prismatic structure, differing in no respect from ordinary 

 Spirifers or Athyrts. 



* In this species, and in Spiriferina spinosa of Norwood & Pratten, 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. WiNCHKLL remarks, of the proposed Genus Syringothyris, that ^'the shell is 

 impunctate in all conditions and under high powers." 



