„ " - LAMELLIBRANCniATA. 601 



Otenoaonta intermedia.] 



Adductor scars subovate, situated immdeiately beneath the ends of the hinge, dis- 

 tinct, the posterior one the deeper and margined on the inner side by an obtuse 

 ridge-like swelling. Small accessory scars have not been observed, 



A single imperfect valve was all I had seen of this species when I first described 

 it. During the summer of 1892, however, I succeeded in collecting an excellent 

 series of specimens, so that I am now enabled to present the shell in all its characters 

 and to point out those which are really distinctive. Compared with G. astartiformis 

 Salter, of which an authentic example is now before me, it differs externally in its 

 greater proportional width, somewhat narrow posterior curve, less convex valves, 

 finer concentric lines and in wanting the coarse wrbikles of growth which seem to 

 be a constant feature of the ventral half in that species. Internally the muscular 

 scars and the denticles of the hinge are about the same in the two species, but the 

 hinge plate is considerably wider in the Minnesota form, while the flat space beneath 

 the denticles of the latter is scarcely represented in Salter's species. Casts of the 

 interior of the two species are not easily distinguished, the only reliable differences 

 between them, so far as observed, being the lesser prominence and more uniform 

 curvature of the anterior margin and the slightly greater convexity of the casts of 

 C. astartiformis. 



Formation and locality. — Upper part of the middle third of the Trenton shales at several localities 

 in Goodhue county, Minnesota. Casts belonging to this species or to C. astartiformis, the latter probably, 

 have been found in the upper part of the Trenton limestone at Minneapolis and at Janesville, Wisconsin, 

 and I have specimens of a very similar, though smaller, form from the upper third of the Trenton shales. 



Ctenodonta intermedia Ulrich. 



PLATE XLir, FIGS. 95-97. 



Tellinomya intermedia Ulrich, 1892. Nineteenth Ann. Rep.G-eol. and Kat. Hist. Sur. Minn., p. 218. 



Shell thin, of medium size, moderately ventricose, rather erect, the" hight a little 

 greater than the length. Outline subtriangular, at the beaks, which, are obtusely 

 acuminate and incurved, forming very nearly a right angle; anterior cardinal margin 

 very gently convex, posterior cardinal edge correspondingly concave, ventral margin 

 together with the curve into the ends forming a semicircle. Ends subequal, the 

 posterior sometimes a little the longest. Umbones full, the remainder of the surface 

 sloping uniformly to the free margins. An obscure sulcus may be detected near the 

 anterior margin, and along the dorsal part of this end the surface descends abruptly 

 to the hinge plate. Surface with strong, closely arranged, thread-like, concentric 

 lines, about twelve in 5 mm. At intervals of about 2 or 3 mm. generally a fold 

 stronger than the rest. 



Casts of the interior exhibit a faint ridge and sulcus in the anterior end, and 



