50 



OHIO FOSSILS 



Z. cincinnatiensis is larger than Z. modesta and has only 5 plications on each side; it is found in 

 the Fairview formation. 



In Plectorthis the hinge is short but almost straight and the sinus and fold are absent. P. 

 plicatella , found in the Fairmount member of the Fairview formation, is characterized by the 

 splitting of the radial ridges. 



Among the shells with more than 40 radial ribs and without sinus and fold, the most easily 

 recognized are the large (1. 5 inches or more wide) Strophomenas and Rafinesquinas. The shell 

 has a long straight hinge line, one valve is convex and the other concave in both genera. In 

 Rafinesquina the valve that has the hole for the pedicle (the brachiopod's organ for attaching to 

 a solid surface) is convex and the other (called the brachial valve) is concave. The convexity is 

 reversed in Strophomena. 



Rafinesquina alternata (fig. 64) has unequal radial ribs, one 

 coarse alternating with several fine ones. It is found in all Cin- 

 cinnatian formations and in some beds is the commonest shell to 

 be found. Rafinesquina alternata fracta of the McMillan is thin 

 and almost flat. 



In all the species of Strophomena (fig. 65), the striations are 

 irregular but not alternating as in Rafinesquina . The many species 

 are hard to characterize. 



Three genera, Resserella , Dinorthis , and Hebertella , have 

 more than 40 radial ribs and a sinus and fold which are rather in- 

 conspicuous. 



We have 3 common species of Resserella 



( Dalmanella of older publications). R. emacerata , 

 found in the Eden, is about 3/4 inch wide, has a 

 rectangular outline, no sinus on the brachial valve; 

 R_. multisecta is about 1/2 inch wide, has a 

 rounded outline, and a sinus on the brachial valve; 

 it is also found in the Eden. R. meeki (fig. 66) 

 is like R. multisecta but larger, 3/4 inch wide, 

 and thicker; it is found in the Richmond. 



Dinorthis and Hebertella are externally very similar and 

 are distinguished especially by internal characters. P. carleyi , 

 confined to the Middle Arnheim, has a peculiar distinguishing 

 feature: the area under the beak of the convex valve is slanted 

 forward; it is 1. 25 inches or more wide. D. subquadrata has a 

 straight area under the beak, a squarish outline, and its coarse 

 ribs split irregularly; it is about the same size as D. carleyi and 

 is found in the Liberty and Whitewater formations. Hebertella also has coarse, irregularly split- 

 ting ribs. Its internal characters distinguish it from Dinorthis . We have 3 common species: 

 Hebertella sinuata is about 1.25 inches wide, has a rectangular outline; it is found in the McMillan 

 formation. IL occidentalis (fig. 67) is about the same size as H. sinuata; it has a characteristic 

 groove on the fold; it occurs in the Maysville and Richmond. H. insculpta is a little smaller (1 

 inch wide) than the preceding two, has sharp concentric ornamentation cutting the radiating ribs; 



it is found in the Waynesville and at the base 

 of the Liberty. 



PELECYPODA. More than 80 species 



Fig. 65 



Fig. 66 



Fig. 67 



belonging to some 25 genera have been des- 

 scribed from the Ordovician rocks of Ohio. 

 Many of these are very rare; in fact, some 

 of them are represented by only a few speci- 



