LAMELLIBRANCHIA TA. 
311 
Anterior muscular impression large and strong, situated just within the 
anterior margin of the shell. 
Three specimens measure respectively 46, 42 and 38 mm. in length, and 
35, 31 and 29 mm. in height. 
As compared with M. bellistriatus it is much larger than the prevailing forms 
of that shell, the basal line is more strongly curved, the post-umbonal slope 
not so well-defined, the posterior extremity usually more curved, and never 
angular as in that species; the striae, except upon the umbo, are never so well- 
defined. In its extremely young state, its surface characters resemble M. bel¬ 
listriatus, and the form is not essentially different, but it is less angular, and the 
umbonal slope less distinctly defined. 
Formation and localities. In the compact arenaceous shales of the Hamilton 
group, at Hamilton, Madison county, and at Summit and Fultonliam, Schoharie 
county; it also occurs in the softer calcareous shales of the group on the 
shores of Canandaigua lake, Ontario county, N. Y. 
Microdon (Cypricardella) complanatus. 
PLATE XLII, FIG. 22; AND PLATE LXXIV, FIGS. 14-19. 
Microdon ? complanatus, Hall. Prelim. Notice Lamellibranchiata, 2, p. 33. 1870. 
Microdon ( Mierodonella ) ( Eodon) complanatus, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: 
PI. 74, figs. 14-19. 1883. 
Shell large, sub-rhomboid-ovate; length more than one-third greater than the 
height; basal margin regularly curved, sometimes straighter on the posterior 
portion; posterior margin obliquely truncate, slightly rounded; cardinal 
margin usually slightly arcuate, often appearing nearly straight. Anterior 
end declining somewhat rapidly from the beaks, and abruptly rounded below. 
Valves moderately convex below, somewhat gibbous in the umbonal region 
and flattened above. The shells are sometimes so compressed that the 
umbonal ridge is obsolescent. 
Beaks at about the anterior fifth, small and not prominent. Umbo prom¬ 
inent. Umbonal ridge usually conspicuous, rounded, extending to the post- 
basal extremity. 
