FOSSILS OF THE CLINTON GROUP. 575 



The shell has in general the same characteristics as that of Orthis 

 fausta. The brachial valve is moderately convex and has a faint sug- 

 gestion of a median sinus posteriorly, owing to the intercalation of 

 radiating plications in this region at a sufficient distance from the beak 

 to cause this suggestion. The pedicle valve is more strongly convex, 

 especially towards the beak. The hinge area is narrow and erect in the 

 brachial valve and considerably higher and outwardly inclined in the 

 pedicle valve. No delthyrium could be detected, although the hinge 

 area was distinct, but this may have been filled up in the specimens 

 found. The breadth of the shell is however relatively greater than in 

 Orthis fausta in shells of the same size, the largest specimen of the 

 variet}- squamosa having a brachial valve with a length of 9 mm. and a 

 width of 15 mm. while in Orthis fausta a valve of the same length would 

 have an average width of about 13 mm. Moreover, for the same sized 

 shell the variety has fewer radiating plications, especially fewer which 

 start from a point near the beak. This difference is well seen in the 

 brachial valve, where in the case of variety squamosa only eight to ten 

 radiating plications seem to reach or almost reach the beak, whereas in 

 Orthis fausta this number of plications varies from twelve to fifteen on 

 the average. The radiating plications of the variety are therefore more 

 divergent, more distant, and seem therefore to be larger and bolder. 

 Lastly, the concentric lines of growth in the variety are more prominent; 

 being lamellose, and the borders being rather strong, and at the same 

 time distinctly elevated above the general surface of the shell, the valves 

 have that coarseness of ornamentation, to which the name squamosa was 

 intended to give expression. The lamellose lines of growth are bent 

 in zigzag fashion over the. plications, but in the type specimen to such a 

 moderate extent that the shell seems ornamented by coarse, fairly 

 regular, concentric lamellae, rather than by zigzaging striae of growth. 

 The greater width, fewer plications, and stronger and more elevated con- 

 centric striae of growth are therefore the distinguishing features of this 

 form. 



This form has a different outline from O. rugaeplicata, Hall and 

 Whitfield, the radiating plications are less prominent, but the concentric 

 squamose striations are at least equally distinct if not more prominent than 

 in the Louisville species. It is also less obese.- The species do not seem 

 closely related. 



Orthis (Herbertella) Daytonensis, Foerste. 



(Plate 25, Figs. 13, a, b, c, d : Figs. 20 a, b ; Fig. 21.) 



When young both valves are moderately and almost evenly convex. 

 With increasing age the brachial valve becomes more and more convex, 

 while the pedicle valve diminishes in convexity, owing to the flattening 

 of anteriorly added portions of the valve, and a somewhat similar flatten- 

 ing of the area added to the postero-lateral angles of the same. In the 



