344 



NORTH AMERICAN INDEX FOSSILS. 



CIX. Rhynchospira Hall. 



Rostrate, subtriangular. Hinge line short and curved. Umbo 

 of pedicle valve incurved; apex truncated by circular foramen. 

 Deltidial plates coalesced ; teeth small, not supported by dental 

 plates. In brachial valve hinge plate produced anteriorly into two 

 flat lobes. Median septum short and sometimes obscure. Surface 

 bearing simple radial plications. Shell punctate. Siluric-Missis- 

 sippic. 



390. R. formosa Hall. Devonic. 



Pedicle valve tapering toward the beak, which is prominent and 

 arched. Brachial valve gibbous, with closely incurved beak. 

 Surface marked by 18 to 23 simple plications, two or three of 

 which are smaller and slightly depressed on the middle of each 

 valve ; these are crossed by fine concentric growth lines which be- 

 come strongly lamellose anteriorly. 



Helderbergian of Maine, New York, Ohio. 



Fig. 441. Homoeospira evax X 2 - (A.f- Fig. 442. Hustedia mormoni (Ind. 

 terHall.) Geol. Survey). 



CX. Homceospira Hall and Clarke. 



Like Rhynchospira but in the brachial valve the crural plates are 

 separated by a linear cardinal process, and a high median septum 

 is present, while the deltidial plates frequently remain uncoalesced ; 

 shell punctate. Siluric. 



391. H. (Retzia) evax Hall. (Fig. 441.) Siluric. 



Ovate, usually longer than wide, gibbous. Both valves some- 

 times bearing a shallow sinus anteriorly. Pedicle valve the deeper, 

 with greatly elevated and incurved umbo. Foramen circular. 

 Surface marked by 16 to 28 radiating plications crossed by fine 

 striae and imbricating, lamellose growth lines. 



Niagaran of Tennessee, Indiana. 



