TRENTON LIMESTONE. 129 



In the young shells of this species, looking upon the ventral valve, the beak of the dorsal 

 valve is distinctly seen projecting beyond the umbo ; but as the shell grows older, this part 

 of the ventral valve becomes gibbous, and projects beyond the line of the beak of the 

 opposite valve. 



Figs. 2 a, b, c, d. e. A series of specimens, illustrating the variations produced by age. 



Figs. 2/, g, h. Profile views of specimens of different ages, showing the gradual increasing rotundity 



of the ventral valve, and also the more acute beak of the dorsal valve as compared to the 



last 

 Figs. 2i,k Front views of young and old specimens. 

 Figs. 2 I, m. Cardinal views of young and old specimens. 

 Figs. 2 », Ventral valves of a young and old specimen. 

 Fig. 2 p. Interior of the dorsal valve, showing the form of the visceral impression, and the radii near 



the margin. 

 Figs. 2 5', r. The two lower figures show a slight difference in the character of the visceral impression. 

 Fig. 2 t. Interior of the ventral valve, showing the medial tooth, which is thicker and stronger than in 



the last species, and is connected with a strong medial ridge which is trifid below. The 



radii are only shown near the margin of the valve ; in which respect, also, it differs from 



the preceding species. 



Position and locality. This is equally abundant with the two preceding and the following 

 one, in many western localities. It occurs at Maysville ( Ky.) , and Cincinnati ; while it is 

 less frequently found at Oxford ( Ohio) , and at Madison ( la.) . 



171. 18. ORTHIS SUBJUGATA ( n. *p.) . 

 Pl. XXXII. C. Figs. 1 a - m. 



Semioval, with the front somewhat produced ; cardinal line nearly equal the width of 

 the "shell ; dorsal area moderately large, triangular ; ventral area narrow, linear ; dorsal 

 valve having its greatest elevation just below the beak, or sometimes at the point of the 

 beak; sides somewhat depressed, often flattened, with a deep broad mesial depression; 

 ventral valve convex, gibbous on the centre and towards the umbo ; surface marked by 

 even rounded strise, which are dichotomous and trichotomous towards the margin ; con- 

 centric lines not very conspicuous ; a few imbricating lines of growth visible towards the 

 margin ; a mesial elevated ridge in older specimens. 



This shell bears considerable resemblance to the last, except that the striae are much 

 finer, and the concentric lines less distinct. The beak of the dorsal valve is less elevated, 

 and the umbo of the ventral valve, in old shells, more prominent than in the preceding 

 species. There is some variation in the length of the cardinal line, as compared with the 

 width of the shell ; but in the one under consideration, it is always proportionally more 

 extended than in the preceding species. The sinus of the dorsal valve is also more distinct 

 in the young shells, and is e<iually conspicuous in older ones. 



[PALiKONTOLOGY.J 17 



