204 



NORTH AMERICAN INDEX FOSSILS. 



anteriorly and with posterior margin abruptly rounded or truncate. 

 Surface marked by fine concentric striae and with faint radiating 



lines. The brachial valve some- 



J^^\ J^)\ i^flk what resembles a Lingula. 

 p^'^Mk k-^^% l((^~^§l Genesee and Naples of New 



^Z^y V^">y \^3y York, Kentucky, Indiana. 



Fig. 237. Schizobolus concentricus, in- 

 terior of pedicle and brachial valves and 

 exterior of pedicle valve, X 2 - 



XIX. Orbiculoidea d'Orbigny. 

 Corneous, subcircular. Ap- 

 ices excentric. Pedicle valve 

 with a narrow pedicle groove beneath the beak extending almost 

 to the margin of the valve ; just behind the beak this groove 

 penetrates through a long tube obliquely to the interior of the 

 valve where it opens near the margin ; in this it differs from Dis- 

 cinisca. Brachial valve larger, depressed conical. Ordovicic- 

 Carbonic. 



48. 0. lamellosa Hall. Ordovicic. 

 Apex nearly central. Surface covered with rather strong sub- 



lamellose concentric growth lines which become more distinct and 

 coarser from the apex outward. 



Trenton and Lorraine of New York, Minne- 

 sota, Ontario. 



49. 0. lodiensis Vanuxem. (Fig. 238.) 



Devonic. 



Small, rarely exceeding one-third inch in 

 diameter. Brachial valve with minute apex. 

 Pedicle valve somewhat abruptly elevated at 

 apex, with linear foramen. Surface covered with fine concen- 

 tric striae and on the anterior half of the shell with faint radiating 

 folds or undulations. 



Genesee of New York, Nevada. 



50. 0. convexa (Shumard). (Fig. 236, d.) Upper Carbonic. 

 Pedicle valve broadly convex with height nearly equal to one half 



the diameter ; apex moderately prominent, situated at about one 

 third the distance from the posterior margin. Surface marked by 

 distinct concentric growth lines. Diameter about one inch. 

 Ohio, Indiana, Missouri, Kansas. 



51. 0. missouriensis Shumard. (Fig. 236, e.) Upper Carbonic. 

 Small, averaging about one fourth inch in diameter. Apex 



Fig. 238. Orbicu- 

 oidea lodiensis, dorsal 

 valve, X 2 « 



