512 bulletin: museum of comparative zoo* 



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

 valve and by intercalation on the brachial. 



Winchell and Schucherl have pointed out how Dinorthis met 

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



striae; how this development was carried further in I), meedri var. 

 germana, and how this form is closely linked with Plaesiomys 8vb~ 

 quadrata. Dinorthis proavita and Plaesiomys svbquadrata show a 

 reversion to ancestral characteristics in the simplicity of the Btriae 

 and the rareness with which interpolation and bifurcation takes place 

 beyond the early neanic stage. 



Length Width Greatest convex it > 



20 mm. 30 mm. 1*> 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 somewhat 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 

 number of striae and is found onlv in the Trenton of Canada. 



