512 bulletin: museum of comparative zoology. 



ing the lateral margins. Striae increase by bifurcation on the pedicle- 

 valve and by intercalation on the brachial. 



Winchell and Schuchert have pointed out how Dinorthis vieedsi 

 developed from D. pectinella var. sweeneyi by the multiplication of 

 striae; how this development was carried further in D. mecdsi var. 

 germ ana, and how this form is closely linked with Plaesioinys sub- 

 qiiadrata. DinortJiis proavita and Plaesiomys subquadrata show a 

 reversion to ancestral characteristics in the simplicity of the striae 

 'and the rareness with which interpolation and bifurcation takes place 

 beyond the early neanic stage. 



Length Width Greatest convexity 



26 mm. 30 mm. 16 mm. 



Locality: — Richmond, Ind. ; Spring Valley, Minnesota; Wilming- 

 ton, Illinois; Warren and Jefferson counties, Missouri; Lattners, 

 Clermont, Iowa; Iron Ridge, Wisconsin; Lake Winnipeg, Canada; 

 Anticosti; Texas. 



Dinorthis proavita Winchell and Schuchert. 



Plate 1, fig. 4. 



Orthis proavita Winchell and Schuchert, Amer. geol., 1 April, 1892, 9, p. 290. 

 Orthis petrae Sardeson, Bull. Minn. acad. nat. sci., 9 April, 1892, 3, p. 332, pi. 5, 



fig. 18-21. 

 Orthis (Dinorthis) proavita Winchell and Schuchert, Geol. Minn., 1893, 3, pt. 1, 



p. 431, pi. 32, fig. 51-57. Whiteaves, Pal. foss. Canada, 1895, 3, pt. 2, 



p. 120; 1897, 3, pt. 3, p. 176. 

 Dinorthis -proavita Schuchert, Bull. 87, U. S. G. S., 1897, p. 216. 



Several specimens of this species were found at Clermont and 

 resemble in all essentials the types from Spring Valley, Minnesota. 

 The striae increase by interpolation on the brachial and by bifurcation 

 on the pedicle-valve. The latter is somew^hat elevated at the umbo, 

 flattened and in some cases concave toward the lateral and anterior 

 margins with a slight mesial elevation. The distinguishing charac- 

 teristic of this form is the brachial-valve which is always convex and 

 possesses a well-pronounced sinus. No other species of Orthidae 

 present at Clermont has the very noticeable sinus of this species. 



Dinorthis proavita and D. iphigenia Billings are superficially similar, 

 but they have the fold and sinus reversed. The latter has a greater 

 nmnber of striae and is found only in the Trenton of Canada. 



