BRACHIOPODA. 311 



rest upon the bottom of the valve ; between them arises a low median septum 

 which may be traced from one-third to one-half the length of the valve, divid- 

 ing a subcordate muscular area, the outer margins of which are distinctly 

 elevated. 



In the brachial valve the crural plates are united to form a bilobed cardinal 

 process. The outer face of this process has not been observed ; on the 

 inner surface it is not continued into a median septum but ends abruptly. 

 Muscular area very faintly defined in the type-species. Internal surface over 

 the pallia! region finely papillose. Shell-substance fibrous, punctate. 



Type, Chonetes reversa, Whitfield. Corniferous limestone.* 



Observations. The peculiar species which is taken as the type of this new 

 division occurs in the fauna of the Corniferous limestone, at Delaware, Ohio, 

 and Cayuga, Province of Ontario. Certain of its peculiarities indicated will 

 not permit its strict generic association with Chonetes, e.g., the reversed con- 

 vexity and surface ornamentation of the valves, and the apparent structure of 

 the cardinal process. In a general sense the relation between Chonostrophia 

 and Chonetes is the same as that between Strophonella and Stropheodonta, 

 Strophomena and Rafinesquina ; but, as in these cases, the reversal of the rel- 

 ative convexity of the valves is not the only difference of importance. Chonetes 

 complanata, Hall, of the Oriskany sandstone, a much larger, more transverse 

 form, also with reversed convexity and with more strongly developed flabelli- 

 form muscular areas, may be placed in the same division until its characters 

 shall be more fully determined. In a different facies of the same fauna is an- 

 other, as yet undescribed congeneric species ; and Mr. Billings has described* a 

 form from an equivalent horizon at Gaspe, Chonetes Dawsoni, In the shaly 

 limestone of the Lower Helderberg series, there occurs a species described in 

 this volume as Chonostrophia Helderbergia, sp. nov. 



* The horizon of this species in Ohio is considered by Mr. Whitfield (Annals of the New York Academy 

 of Sciences, vol. ii, p. 212 ; 1S82), the equivalent of the Marcelliis shales of New York, on account of its 

 association with a few species of the New York fauna. The character of the sediments is, however, alto- 

 gether different, the rocks at Delaware, Ohio, \)e'mg largely calcareous. They lie at the top of the Cornifer- 

 ous series hi that State and are included in this series by Pi-ofessor Orton, in his First Report of the Third 

 Geological Survey of Ohio, 1890. The association of the species in the Province of Ontario is invariably with 

 characteristic forms of the Corniferous fauna. 



* PaljEOzoic Fossils, vol. ii, p. IS. 1874. 



