r..\MKl.l.ini: \N< in \ r\ ''."1 



OtMMdoaU laUnMMllk.1 



Adductor scar- Milimate. situated immdeiately beneath the ends of the hinge, 

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

 ridge-like swelling. Small accessory scars have not heen observed. 



A .sinirl.' imperfect vahe was all 1 had seen of this species when I first described 

 it. liming tin 1 Mimmer of ls'.'J. however, I succeeded in collecting an excellent 



- of >p,.| 'iinen-, .-o that 1 am now enabled to present the shell in all its characters 

 and to point out those whicli arc really di-t ine.tive. Compared with C. asturtiformis 

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

 -iv.tter proportion.il wi.lth, somewhat narrow posterior curve, less convex valves, 

 linn- concentric lines and in wanting the coarse wrinkles 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 

 hinu'e plate is considerably wider in the Minnesota form, while the Hat space beneath 

 the denticles of the latter is scarcely represented in Sailer'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. 



Furmation and locality. Upper part of the middle third of the Trenton Abates at Mvoral localities 

 in Ooodhur < unty. Minnesota. Casts belonging to this species or to C. (utartiformit, the latter probably, 

 have been found In the upper part of the Trenton limestone at Minneapolis and at Janeavllle, Wisconsin, 

 and I have specimens of a rery similar, though smaller, form from the upper third of th<- Trvnton shale*. 



KNODONTA INTERMEDIA Ulrich. 

 PLATE XLII. KIOS. 06-07. 



Trllinomya intermedia I'I.KI. ii. 1882. Nineteenth Ann. Rep. Oeol. and Nat I! - .r. Minn , p. 218. 



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

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

 acuminate ami incurve'!, 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. Knds subequal, the 

 posterior sometimes a little the longest I' in bones full, the remainder of the surface 

 sloping uniformly to the free margin-. An oK-cure sulcu.s may be detected near the 

 anterior margin, ami along the dorsal part of this en>l the surface descends abruptly 

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

 lines, about twelve in > mm. At intervals of about : mm. generally a fold 



stronger than the rest. 



Casts of the interior exhibit a faint rid^e and sulcns in the anterior end, and 



