538 THE PALEONTOLOGY OF MINNESOTA. ,n„.tnH„ t. , 



[Oyrtoaouta ampla. 



of moderate strength; cardinal teeth three in each valve, sub-equal, curved and 

 rather oblique; posterior teeth slender, two or three in each valve. Surface of shell 

 with somewhat irregular concentric lines of growth. No trace of these are to be 

 seen in casts of the interior. 



It is possible that the casts above described really belong to C. huronensis. 

 Although I have compared them with an authentic example of that species, labelled 

 by Billings himself as from the original locality for the species, I could not satisfy 

 myself. The Wisconsin casts are certainly distinct from this specimen, having 

 smaller umbones and shorter anterior end, but the latter also does not agree with 

 Billings' figures. Very likely the illustrations are not entirely trustworthy. 



Compared with C. subovata, the species is distinguished by its shorter, narrower, 

 and less distinct anterior end, comparatively greater length, less produced and more 

 oblique cardinal teeth, and more distinct muscular and pallial impressions. That 

 species also attains greater size. 



Fornication and locality.—" Lower Blue beds" of the Trenton at Janesville and Beloit, Wisconsin. 

 Mus. Beg. No. 8323. 



CXKTODONTA AMPLA, W. Sp. 

 PLATE XXXIX, PIG. 34. 



In the outline this species resembles C. subovata and C.'janesvillensis very closely. 

 It is known only from casts, but these are distinguished at once by the oblique ridge 

 running from the beak toward the posterior third of the base. On the anterior side 

 the surface descends sharply from the ridge into an unusually wide flattened space. 

 C. janesvillensis is also narrower anteriorly and- relatively more convex. Another 

 species with which it is to be compared is the Galena form described by Meek and 

 Worthen as G. obliqua. The outline of that species is different being narrower in 

 front and more produced in the postero-basal region, giving the shell a more erect 

 appearance. Its valves are also a little more convex, C. glabella is shorter. In the 

 associated forms of Vanuxemia the anterior adductor scar is much more sharply 

 defined. 



Formation and locality. — Trenton limestone, Cannon Falls, Minnesota. 



Ctktodonta billingsi, n. sp. 



PLATE XL, FIGS. 2—6. 



Cypricardites ventricosus Wtiitfleld, 1882 Geol. Wis., vol. iv, p. 209, pi. 5, flg, 9. 



Shell of medium size or less, transverse, obliquely ovate, highest in the posterior 

 half; valves strongly ventricose in the umbonal and central regions. Hinge line 

 at least two-thirds the length of the shell, slightly arcuate, posteriorly declining 



