602 THE PALEONTOLOGY OF MINNESOTA. 



(.Ctenodonta alt a. 



two sharply defined muscular scars and pallial Hue in each valve.* Hinge plate 

 rather narrow, the teeth numerous, over thirty, as usual very small centrally, grow- 

 ing larger gradually towards the ends of the hinge. 



This species is distinguished from C. astartiformis Salter, by its larger size, greater 

 width, more erect form, and comparatively coarse and regular concentric lines. C. 

 compressa is not so convex, especially in the umbonal region, has more pointed beaks 

 and much finer striae. 



Formation and locality. Not uncommon in the middle division of the Galena at WykofT and other 

 localities in Fillmore county, Minnesota. 



CTENODONTA ALTA Hall. 



PLATE XLII. FIGS. 03 and 94. 



Tellinomya alta HALL, 1861. Kep. Supt. Geol. Sur. Wis., p. 27; MEEK and WORTHEN, 1868, Geol. 



Sur. 111., vol. iii, p. 309. 



Shell (internal cast) small, rather strongly convex, nearly erect, subtriangular, 

 the length, hight and convexity, respectively, about 11.2, 11.5 and 6.3 mm.; base 

 broadly rounded, semielliptical; beaks elevated, nearly central, arching slightly back- 

 ward; anterior and posterior sides nearly equal, sloping abruptly from the beaks at 

 an angle of about 85, the anterior dorsal outlines very gently convex, the posterior 

 correspondingly concave; beneath the ends of the hinge the outline on both sides 

 curves rapidly into the base. Muscular scars larg-e and comparatively distinct, the 

 posterior one nearly rounded, the other more oval; the anterior one lies in the wider 

 lower end of a shallow sulcus which may be traced almost to the beaks. The hinge, 

 Prof. Hall says, is marked by from twenty to twenty-five very small curved teeth on 

 the anterior (posterior) side and from ten to fifteen on the posterior (anterior) side. 



This rare species is a little smaller, not quite as high, more erect and less convex 

 in the basal outline than C. intermedia. In the latter, as well* as in all the other 

 species of this section of the genus, save C. recurva, the anterior half of the outline 

 is more uniformly rounded. 



Formation and locality .The specimen described by Meek and Worthen came from the Galena 

 near Mount Carroll, Illinois, while Hall's original type is from, presumably, an equivalent horizon at 

 Dodgeville, Wisconsin. The specimen here used, which is precisely like the Illinois example, is from 

 (.In- shaly lower beds of the Galena near Fountain, Minnesota. 



In the original description it is stated that a small pair (if soars is situated above the posterior adduet.or impres- 

 sions. Tliis stateiiMTit I nou helir\e rests upon faulty observation. 



