LAMELLIBRANCHIATA. 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. bellisiriahis 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 Fultonham, 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. 



Mlerodon t eompianatus. Hall. Prelim. Notice Lamellibranchiata, 2, p. 33. 1870. 



Mierodim (Mlerodonella) (Eodun) 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. 



