BRACHIOPODA. 31 



portion of the inner layers ; and on different portions of the valves the feature 

 is differently developed. In Spirifer plenus, Hall, of the Burlington limestone, 

 we have an example of a true Spirifer with the apical callosity in about the 

 condition of development exhibited by the Devonian species, S granulosus; that 

 is, with the syringothyroid tube incipient, but in which the shell is punctated 

 for its entire thickness. 



Of the Ostiolati, or syringothyroid Spirifers, the American faunas possess 

 a considerable representation. The type was not specialized until the opening 

 of the Devonian, and with the exception of the species just cited, S. plenus, 

 and 8. negledus, Hall, of the Keokuk group, it passt-d into Syringothvris at the 

 close of the Devonian period. It is, therefore, a Devonian spiriferoid type of 

 preeminent importance. 



A member of this group which represents a form of exterior somewhat un- 

 like that of a'll the other species here associated with it, is Spirifer acuminntus, 

 Conrad ; characterized by its broad duplicate lateral plications, a feature of the 

 rarest occurrence among the Ostiolati. This specific type, however, is wide- 

 spread, being represented in the Devonian faunas of the continent of Europe 

 by the shell known as S. cuUrijugatus, F. Roemer.* 



In the Upper Helderberg limestone are the following species of Ostiolati : 



S. perextensus, Meek and Worthen. S. Manni, Hall. 



S. macrothyris, Hall. S. acuminatus, Conrad. 



S. angustus, Hall. 



In the Hamilton group : 



S. angustus. Hall. S. granulosus, Conrad. 



S. Marcyi, Hall. S.fornax, Hall. 



& acuminatus, Conrad. S. eurytines, Owen. 



S. audaculus, Conrad. S. Parryanus, Hall. 



S. ligus =pennatus, Owen. S. asper. Hall. 



S. Wortheni, Hall. S. Macbridii, Calvin. 



*See RoBMKR, Das rhein. Uebergangsgebii-ge, pi. Iv, fig. 4; Schuh., Beschr. Eifel. Brach., pi. xxxiii, 

 tig. 1. Most instructive figures are given by QiiENSTEDT, Brac-hiopoilen, p!. lii, figs. 19-21. There is little 

 doubt of tlie specific identity of the shells passing under these two names, and Roemer's designation must 

 eventually give way to the earlier one of Conrad. 



