574 GEOLOGY OF OHIO. 



plate, as far as its anterior end of attachment, being thickened, sometimes 

 more or less abruptly, so as to form a sort ot recurved cardinal tooth 

 resting. posteriorly ucon a thin plate as a support. 



The valves are thin, readily showing the radiating plications inter- 

 iorly, except where they have been thickened at the muscular areas. In 

 the brachial valve this thickening is almost confined to the production of 

 a low median elevation, which separating the adductor impressions 

 scarcely extends the full distance. From, this branch off perpendicularly 

 two narrower elevations, obsolete in the great majority of specimens, 

 and separating the anterior from the posterior pair of adductor impres- 

 sions. The posterior of these impressions is very small, and the anterior 

 ones though of twice the diameter are also small. They cannot be said 

 to have any anterior or lateral boundaries. The muscular area of the 

 pedicle valve is much better defined owing to the thickening of the shell 

 which has taken place over its area, this thickening ceasing at the ante- 

 rior boundary of the scars, where it is also the most developed. The an- 

 terior end of the area has a rounded or slightly trilobate form, the area 

 itself being often traversed by fine striae parallel to its anterior outline. 

 Laterally where the shell of the area is thinnest the radiating plications 

 show through; two larger striae, also diverging, border the strongly thick- 

 ened elongate triangular median portion, occupying the larger portion 

 of the muscular area. If this be regarded as the adductor muscular im- 

 pression, the smaller lateral portions of the area will be the diductor 

 muscular impressions. Bordering the latter on the sides is a low ridge, 

 rising posteriorly into acute erect, platelike walls supporting the teeth, 

 as already described. The shell structure is fibrous-impunctate. 



The typical forms of Orthis fausta differ from Orthis Nisis of Hall 

 and Whitfield from the Niagara at Louisville, Kentucky, in having 

 coarser radiating plications than the type of that species, and a less ele- 

 vated cardinal area in the pedicle valve. But the two forms are evidently 

 very closely related and the Clinton species is regarded as phylogeneti- 

 call} r the immediate precursor of the Niagara O. Nisis. Orthis rugcz- 

 plicata is a much coarser species. 



Orthis fausta is found in the upper shaly courses at Soldiers' Home, 

 Huffman's, Fauver's, Centreville, and probably in the limestone at 

 Brown's Quarry. 



Orthis fausta, var. squamosa, var. nov. 



(Plate 37 A, Figs. 19 a, b.) 



Associated with Orthis fausta at the Carrollton Pike Quarry, about 

 half a mile west of the Soldiers' Home, near Dayton, Ohio, and south 

 of the road, is a distinct form, readily recognized, but evidently related 

 to the former, and hence here described merely as a variety. Its internal 

 features are not known; but when found these may raise the variety to 

 the dignity of a species. 



