LAMELLIBRANCHIATA. 309 



Six characteristic specimens have the following dimensions, respectively : 

 Length 46, 37, 35, 37, 28 and 38 mm.; height 27, 22.5, 25, 28, 21 and 31 mm. 



This species differs from M. gregarius in its broader cardinal slope, less 

 oblique posterior margin, larger size and stronger surface striae. M. complanatus 

 is a larger and more elongate form, the posterior and anterior extremities more 

 regularly curved, the striae irregular and fascicled, the beaks more elevated and 

 the posterior margin more obliquely truncate. 



Formation and localities. In the shales of the Hamilton group throughout the 

 range of this formation in the State of New York, but more abundant in the 

 eastern and central portions ; also from the same horizon, near Cumberland, 

 Md. ; and in the Chemung group, near Elmira and Ithaca, N. Y. ; and from 

 Susquehanna and Tioga counties. Pa. 



MiCRODON (CyPRICARDELLA) GREGARIUS. 



PLATE LXXIU, FIGS. 1-6 ; AND PLATE LXXIV, FIGS. 1-4. 



MicTodon gregaria, Hall. Prelim. Notice Lamellibranchiata, 2, p. 32. 1870. 



Mlerodnn (Microdonella) (Eudon) greganus. Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 



73, figs. 1-6. 1883. 



Shell small, rhomboid-ovate; length less than one-third greater than the 

 height; basal margin regularly curving, sometimes nearly straight toward 

 the post-basal angle ; posterior margin obliquely sub-truncate, slightly curv- 

 ing; cardinal margin gently arcuate or nearly straight. Anterior end 

 declining abruptly from the beaks, with the extremity narrow and abruptly 



rounded. 



Valves depressed-convex, more convex at the umbo and on the umbonal 



ridge. 



Beaks situated at less than the anterior third of the length, small, low, 

 closely appressed, scarcely rising above the hinge-line. Umbonal ridge dis- 

 tinct, but not strongly defined, continuing to the post-basal angle. 



Test thin, marked by fine, concentric striae, which are somewhat fasciculate 

 on the anterior portion of the shell. 

 40 



