310 
PALAEONTOLOGY 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 ; PLATE LXXIII, FIGS. 23-30 ; AND PLATE LXXIV, FIGS. 20, 21. 
Microdon tenuistriata, Hall. Prelim. Notice Lamellibranchiata, 2, p. 32. 1870. 
Microdon ( Microdonella ) ( Eodon) tenuistriatus, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: 
PI. 73, figs. 23-27, 29, 30 ; pi. 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 stria?, which are at intervals 
raised into lamelliform undulations. In some specimens the strife upon the 
umbonal region are very uniform, thread-like and equal throughout. 
