58 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



margin of the notothyrial platform. The dorsal mus- 

 culature has not been observed. 



Geologic range. — Middle and Upper Ordovician 

 (Chazy? to Maysville) of North America. 



name, but their analysis of the group is inadequate. 

 They define Plectorthis as follows (pp. 194, 195): 



While it retains the strong external ribs of the typical 

 Orthis, these are not invariably simple (O. fissicosta, Hall; 



Table 4 



PLECTORTHINi?: 



Schizophorella 



Doleroides 



CyCLOCCELIIN^ 



Cyclocoelia f 



ORTHOSTROPHIINiE 



Mimella 



^ PLATYSTROPHUNiE 



FlNKELNBURGIIDit 



Orusia 



American Species 



Plectorthis tequivalvis (Hall) 1847 



P. t^quivdvis latior Foerste 1910 



P. leguivak'is fervagata Foerste 1910 



P. fsskosta (Hall) 1 847 



P.jamesi (Hall) 1861 



P . neglecta {]amei) 1879 



P.fUcatella (Hall) 1847 



P. flicatella trentonensis Foerste 1910 



P. ? sinuatis Wilson 1926 



P. trif Heal ella (Meek) 1872 



? P./ioUemWWhrd 1928 



? P. exjoliata (Raymond) 1905 



European Species 

 ? Orthis scotica McCoy 



Discussion. — Hall and Clarke recognized the 

 uniqueness of the group of shells characterized by 

 Orthis flicatella Hall by giving this division a separate 



O. triflicatella. Meek; O. tequivahis, Hall, not Davidson; 

 O. Jamesi, Hall) ; the cardinal area of the pedicle-valve is 

 comparatively low and the valves are subequally convex. 

 In the interior the character of the muscular scars, dental 

 lamella and cardinal process is essentially the same as in 

 Group I [=^ Hesferorthis^, and the minute structure of 

 the shell seems to be in precise agreement with that of 

 O. calligramma [= Orthis s. s.\, though no evidence of 

 tubulose costs: has been observed. In Orthis Jamesi, which 

 is placed in this association, there is occasionally a deviation 

 toward the resupinate contour exemplified in the Groups 

 IV and V [= Pltesioniys and Hebertella\. 



In defining this group Hall and Clarke evidently 

 had in mind its variations from their Orthis s. s., which 

 was based chiefly on Orthis tricenarta [= our Hesfer- 

 orthls^. The interareas are rather different from those 

 of Hes-perorthis but do actually resemble those of Orthis 

 s. s. as here defined in the rather prominent incurva- 

 ture of the ventral interarea. Hall and Clarke err 

 when they state that "in the interior the character of 



