TRENTON LIMESTONE. 
125 
166. 13. ORTHIS INSCULPTA ( n. sp .). 
Pl. XXXII. Figs. 12 a, b c. 
Shell resupinate ; dorsal valve depressed-convex ; beak elevated, not incurved ; cardinal 
line less than the width of the shell; area short; surface marked by fine elevated radii, 
which are bifid on the umbo, and again regularly bifid or trifid towards the margin of the 
shell ; transversely marked by strong elevated concentric lines, which are particularly 
prominent between the radii, giving an indented or sculptured appearance to the surface ; 
interior of the dorsal valve showing marks of the visceral impression, which is peculiarly 
sculptured. 
This species, of which a single valve only has been seen, is so peculiarly marked as not 
to be mistaken among all the other species of Orthis in the lower rocks. The cavity under 
the foramen is large and deep, marked on its lower and outer extremities by two pointed 
sacs, and margined by a thickening of the shell around : the visceral impression on the 
outside of this cavity is of a double auricular form, and sculptured by curved elevated lines. 
The inner margin of the shell is impressed by the outer radii. 
This very peculiar form is usually associated with the following forms in the Blue lime¬ 
stone of Ohio. Its very beautiful sculpture, both internally and externally, is sufficient to 
distinguish it from all other species in the rock. 
Fig. 12 a. Dorsal valve, the beak imperfect, b. Interior of the dorsal valve, c. Dorsal area. 
Position and locality. Same as the preceding and following species. 
167. 14. ORTHIS DICHOTOMA (71. sp.). 
Pl. XXXII. Figs. 13 a, b. 
Suborbicular ; valves almost equally convex ; cardinal line less than the width of the 
shell; dorsal area rather large, compared with the size of the shell; beak extended, not 
incurved ; greatest elevation of the dorsal valve below the beak, and opposite the cardinal 
line ; ventral valve uniformly convex ; surface marked by about 26 well defined rounded 
radii on each valve, which uniformly bifurcate about half way from the beak to the base 
of each valve ; radii indistinctly crenulated by obscure concentric lines, which are more 
distinct on the intermediate spaces. 
This is a neat symmetrical shell, which, although not very conspicuous, is nevertheless 
clearly distinct from any other species. The radii are well defined and regularly dichotomous, 
which, with the moderate and nearly equal convexity of the two valves, are characters not 
observed in any other species. It approaches in some respects to 0. cequivalvis , but is less 
convex, and the beaks more unequal. It likewise approaches in some respects to O.Jlssicosta, 
but the radii are more numerous, and the beak not incurved : the entire shell is also less 
robust in habit. 
Fig. 13 a. Ventral valve, showing the projecting dorsal beak. b. Profile view of the same. 
Position and locality. This species occurs at Cincinnati (Ohio), associated with many of 
the preceding and succeeding species. 
