458 THE PALEONTOLOGY OF MINNESOTA. 



ll'latystropliia V>ifor;it;i. v;ir. crash!). 



PLATYSTROPHIA BIFORATA, var. CRASSA James. 



PLATE, XXXIII, FIGS. 53-54. 



1873. Var. 3. Orthis (Platystrophia) dentata ?? MEEK (non PANDER). Palaeontology of Ohio, vol. i, 



p. 117, pi. x, flg. 3. 



1874. Orthis (Platystrophia) crassa JAMES (non LINDSTROJI). Cincinnati Quarterly Journal of 



Science, vol. i, p. 20. 



1875. Orthis dentata MILLER. Ibidem, vol. ii, p. 27. 



1889. Orthis centrosa MILLEK. North American Geology and Palaeontology, p. 356. 

 1892. Platystrophia crassa HALL. Palaeontology of New York, vol. viii, pt. i, pp. 223. 



This variety can be distinguished readily by its short hinge-line, causing the 

 shell to be as wide as long, and its very gibbous valves. This species occurs sparingly 

 in the Hudson River group at Spring Valley, Minnesota, and differs from those found 

 at Cincinnati, Ohio, in having about three more much less elevated costse on each 

 side of the fold and sinus. The muscular scars and other interior characters are 

 more denned than in southern specimens. 



Since this variety is now referred to the genus Platystrophia, the name crassa 

 James will not conflict with Orthis crassa Lindstrom, I860.* The latter is said to be 

 related to 0. elegantula Dalman, and is therefore referable to Prof. Hall's subgenus 

 Dalmanella. 



Mus. Reg. No. 5543. 



Order TELOTREMATA, Beecher. 



Family RHYNCHONELLID^E, Gray. 



Genus RHYNCHOTREMA, Hall. 



I860. Rhynchotrema, HALL. Thirteenth Report, New York State Cabinet of Natural History, p. 68, 



flgs. 7-14. 

 1883. Rhynchotrema, WAAGEN. Palasontologica Indica, ser. xiii, vol. i, p. 410. 



Ehynchonella is an extensive genus, if all the species are admitted that are 

 currently referred to it. It then has its beginning at the base of the Lower Silurian, 

 continues through all the subsequent ages, and is represented at present by five 

 living species. Several names have been proposed by authors for the earlier forms, 

 but none of them have come into general use. 



Rhynchotrema will be employed for those early rhynchonelloid species having a 

 prominent cardinal process between the crural plates of the dorsal valve. This 

 process is very well developed in all Lower Silurian species of so-called Rhynchonella 

 of which the interior has been examined. 



* Gotland's Brachiopodeu, p. 396, 1860. Also Davidson's Mono. British Sil. Brucli., p. 213, pi. xxvii, tigs. 17-19. 



