310 PALEONTOLOGY OF NEW YORK. 



Anterior muscular impression not strongly developed, placed just within 

 the anterior border of the shell. 



Three specimens measure respectively 29, 27 and 22 mm. in length, and 

 22, 20 and 16 mm. in height. 



This shell, in its general form, resembles M. bellistriatus, but the posterior 

 end is always more curving, and the margin more distinctly directed forward, 

 the post-cardinal slope narrower, and the striae are always finer and less regu- 

 lar. As compared with M. tenuistriatus it is usually a little larger proportion- 

 ally ; the hinge-line is shorter, the beak situated a little nearer the anterior 

 end and more obtuse. 



Formation and localities. In the flaggy layers of the upper part of the Ham- 

 ilton group, at Jefferson, Schoharie county ; in shaly sandstones of the same 

 group, between Jefferson and Summit ; and in the lower part of the Chemung 

 group, at Cortland, Ithaca, Elmira and Portville, N. Y. ; and Tioga county. Pa. 



MiCRODON (Cypricardella) tenuistriatus. 



PLATE XLII, FIG. 16 ; l-LATE LXXIII, FIGS. 23-30 ; AND PLATE LXXIV, FIGS. 20, 21. 



MicrodoH tenuiitriata. Hall. Prelim. Notice Lamellibranchiata, 2, j). 32. 1870. 



Microdon (Mierodoivella) (Hodon) tenimtriatus, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: 



PI. 73, figs. 23-27, 29, 30 ; j)!. 74, figs. 20, 21. 1883. 



Shell large, quadrangularly sub-ovate ; length one-fourth greater than the 

 height ; basal margin regularly curving ; posterior margin sub-truncate, from 

 oblique to nearly vertical and gently curving. Cardinal line gently arcuate. 

 Anterior end rapidly declining from the beak and abruptly rounded below. 



Valves depressed-convex below, rising into moderate convexity in the 

 umbonal region. 



Beaks at about the anterior third, small and appressed, rising a little 

 above the hinge-line. Umbonal slope scarcely defined, extending in a slightly 

 arching direction to the post-basal extremity. 



Test thin, marked by fine, unequal, concentric striae, which are at intervals 

 raised into lamelliform undulations. In some specimens the striae upon the 

 umbonal region are very uniform, thread-like and equal throughout. 



