Hall.] 250 i^'^y- 



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 

 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 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 

 scarcely at all in S. increhescens and S. opima. 



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

 this is true of all those I have examined, though in some of the 

 species of the higher formations I have observed characters which, 

 with imperfect means of investigation, seem to be irregularly dis- 

 tributed 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 texture, 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 difl'er- 

 ing from that of Spiriferina as given by Dr. Carpenter. The 

 intermediate spaces have a distinctly and beautifully fibrous or pris- 

 matic structure, differing in no respect from ordinary Spirifers or 

 Athyris. 



It should be observed, in this connection, that the dental plates, 

 both in Spirifera alta and S. textus, reach to the bottom of the 

 cavity, and partially surround the muscular impression, which is pre- 

 cisely like that of ordinary Spirifers. 



* In this species and in Spiriferhui spiiiosa 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. Winchell remarks, of the proposed Genus Syringothyris, that " the shell 

 is impunctate in all conditions and under high powers."' 



