94 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



Ventral interior. — Teeth strong, with shallow sock- 

 ets; crural fossettes oblique; dental plates obscure in 

 old shells, more prominent in young ones, not reced- 

 ing; umbonal cavities shallow; muscle area quadrate 

 in outline, anteriorly bilobed, adductor scars small, the 

 two forming a central oval field divided by a low 

 median elevation; diductor scars diverging, expanded 

 in front, so as to enclose the adductors; adjustor scars 

 prominent, about one-half the length of the diductors; 

 pedicle callist triangular; pallial markings prominent, 

 consisting of two main trunks given off from the 



Geologic range. — Middle and Upper Ordovician 

 (Chazy to Richmond). 



American Species of Dinorthis s. s. 



Dinorthis atavoides Willard 1928 



D . interstriata 'W\]\d.rdi 1928 



D. fectinella (Emmons) 1842 



D.p-oavita (Winchell and Schuchert) 1892 



D. quadriplicata Willard 1928 



Orthis semiovdis Hall 1 847 



O.szveeneyi N. H. Winchell 1881 



Retrorsirostra 



M, 



anon 



Pla:siomys <■ 



Pionorthis 



^Multicostella 



Palaeostrophomena 



ORTHIDi€ 



antero-lateral extremities of the diductor scars; these 

 trunks are short and soon bifurcate, sending one sub- 

 sidiary postero-laterally, the other anteriorly; both of 

 these send off many minor branches. Anterior margin 

 finely crenulate. 



Dorsal interior. — Notothyrial cavity shallow ; brach- 

 iophores orthoid, diverging, unsupported except by ad- 

 ventitious deposit, bluntly pointed ventro-distally ; sock- 

 ets defined by the sloping outer face of the brach- 

 iophore and an excavation beneath the palintrope; 

 cardinal process strong, crenulated on the postero-ven- 

 tral face, the crenulation in old shells giving a bilobed 

 appearance; median ridge short, extending less than 

 one-half the length of the shell; posterior adductor 

 scars smaller than the anterior pair, separated, in some 

 specimens, by curved oblique lines at an angle to the 

 median ridge. 



Foreign Species of Dinorthis s. s. 



Orthis fabdlulum Sowerby 1839 

 O. (Dinorthis) thakil Salter 1865 

 O. (D.) thakil convex a Salter 1865 

 O. (Z>.) thakil trifida Salter 1865 

 ? O. (D.) annamitica Mansuy 



Placed from the 

 literature 



American Species of PljEsiomys 



Dinorthis Columbia WWson 1927 



D . if higenia CWiW'mgs) 1862 



D. meedsi (Winchell and Schuchert) 1892 



D. meedsi arctica Schuchert 1900 



D. meedsi germana (Winchell and Schuchert) 1892 



D.rockymontana'^'^soxi 1927 



D . subquadrata (Yi^V^) 1847 



D. transversa Willard 1928 



D.ulrichi Foerste 1909 



Orthis anticostiensis Shaler 1887 



