BRACHIOPODA. 181 



Observations. These characters, it will at once be remarked, are rhyncho- 

 nelloid, but are essentially primitive in all respects. The type of structure, if 

 strictly interpreted, does not appear to have been a prolific one. The 

 associates of Atrypa dubia in the Chazy fauna, namely, A. plena. Hall, and A. 

 altilis, Hall, are larger, more coarsely plicated shells, with a short median sep- 

 tum in the brachial valve, and a stronger development of the crural bases. 

 They evince a higher development of rhynchonelloid characters and perhaps 

 may be regarded with more propriety as early representatives of the large 

 division here termed Camarot<echia, than as congeneric with Protorhyncha dubia. 

 The Poramboniles Ottawaensis, of Billings, from the Black River limestone 

 of the Pauquette Rapids, appears to be a representative of this structure. 

 The relations of Protorhyncha with the genus Orthis are evinced in the 

 tendency to the formation of a cardinal area, the usually open delthyrium at 

 maturity, and in the short, blunt hinge-processes. 



Genus ORTHOR H YN C HUL A, gen. nov. 



PLATE LVI. 

 1889. Orthis, Nettelroth. Kentucky Fossil Shells, p. 41, pi. xxxiv, fij^s. 7-13. 



Shells rhynchonelloid in contour ; hinge-line short, straight, extending for 

 about one-third the transverse diameter of the valves. A true cardinal area is 

 present on both valves, that of the pedicle-valve being considerably the broader, 

 erect, often incurved. Each valve also possesses a distinct triangular delthy- 

 rium, that of the pedicle-valve, according to the evidence at hand, never being 

 in any degree closed by deltidial plates. External surface strongly and simply 

 plicated, the median fold and sinus being well developed. On the interior, the 

 pedicle-valve possesses blunt teeth which rest upon the laterally thickened 

 walls of the valve and are not supported by lamelloB. Between, and slightly 

 in front of these lies a short, subquadrate muscular scar. The brachial valve 

 possesses a linear cardinal process, on either side of which are two discrete 

 crural plates, sharply concave on the upper surface and diverging anteriorly 

 for a considerable distance. 



Shell-substance fibrous, impunctate. 



Type, Orthis Linmyi, Nettelroth. Hudson River group. 



