TRENTON LIMESTONE. 125 



166. 13. ORTHIS INSCULPTA (n. fp.). 



Pi,. 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 (n.*i?.). 



Pl. XXXII. Figs. 13 o, 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 VtJve uniformly convex ; surface marked by about 26 well defined rounded 

 radii on each valve, which uniformly bifurcate about half way from the beak ao 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. aquivalvis, but is less 

 convex, and the beaks more unequal. It likewise approaches in some respects to 0. fissicosta, 

 but the radii are more numerous, and the beak not incurved : the entire shell is also less 

 robust in habit. 



Fig. 13 a. Ventral valv, showing the projecting dorsal beak. J.' Profile view of the same. 



Position and locality. This species occurs at Cincinnati (Ohio) , associated with many of 

 the preceding and succeeding species. 



