Maryland Gkological Suuvky 303 



sal valve slightly convex near the umbo and flattened toward the front; a 

 small fold begins about one third of the length from the nrabo, extends 

 to the front of the valve, and near the front is limited on each side by a 

 shallow furrow. Surface marked by concentric and very fine radiating 

 striae. 



This shell is similar to a minute form of Ainhococlia umbonain 

 (Con.) ; but it differs from that species in its constant small size and the 

 sharper sides of the sinus. The specimens occur in large numbers form- 

 ing a large proportion of a thin calcareous stratum and they are uniformly 

 small and similar to the specimen described. The material was shown 

 Dr. J. ]\I. Clarke niul it is his opinion that the constant snuill size and 

 angular sinus entitles this form to rank as a species. 



Hall described a form from the Marcellus shale as Orthis nucleus 

 (Geol. N. Y., pt. iv, p. 181 and fig. 8 on p. 180), which later was termed 

 AiiiJ)ocuclia nucleus (Thirteenth Eeport on the State Cabinet p. 71), and 

 finally made a synonym of Amhocoelia unthonata (Pal. X. Y., vol. iv, pp. 

 259, 260), that apparently approaches this species in many respects. 

 Specimens of this form have not been seen by the writer, but it is be- 

 lieved that the species here described is uniformly much smaller. It is 

 to be noted, howevei', that the description of Amhocoelia nucleus states 

 that the sinus " is often very conspicuous, being narrow and sharply de- 

 pressed " (Pal. IST. Y., vol. iv, p. 260) ; but in the original description is 

 the statement that the upper [dorsal] valve is flat (Geol. N. Y., pt. iv, p. 

 181) although later it is stated that a diminutive form similar to the 

 A. nucleus of the Marcellus shale "sometimes occurs in the compact cal- 

 careous beds of the Hamilton group in great numbers .... and in these 

 -the dorsal valve is more convex than usual" (Pal. N'. Y., vol. iv, 

 p. 260). 



^4 virginiana is also somewhat similar to the form which Dr. Grabau 

 first described as variety nana of A. umhonala (Sixteenth An. Pep. State 

 Geologist [IST. Y.], 1899, p. 2?G, figs. 3-7 on p. 277) from near the base 

 of the Hamilton shales on the shore of Lake Erie in western New York 

 and which he later raised to the rank of a species (Bull. Buffalo Soc. 

 Nat. Sciences, vol. vi, 1899, p. 217, fig. 126 on p. 218). Finally. :\liss 



