458 THE PALEONTOLOGY OF MINNESOTA. 
(Platystrophia biforata, var. crassa. 
PLATYSTROPHIA BIFORATA, Var. CRASSA James. 
PLATE, XXXIII, FIGS. 53—54. 
1873. Var. 3. Orthis (Platystrophia) dentata ?? MEEK (non PANDER). Paleontology of Ohio, vol. i, 
p. 117, pl. x, fig. 3. 
1874. Orthis (Platystrophia) crassa JAMES (non LINDSTROM). Cincinnati Quarterly Journal of 
Science, vol. i, p. 20. 
1875. Orthis dentata MILLER. Ibidem, vol. ii, p. 27. 
1889. Orthis centrosa MILLER. North American Geology and Paleontology, p. 356. 
1892. Platystrophia crassa HALL. Paleontology 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 cost on each 
side of the fold and sinus. The muscular scars and other interior characters are 
more defined than in southern specimens. 
Since this variety is now referred to the genus Platystrophia, the name crassa 
James will not confilet with Orthis crassa Lindstrém, 1860.* The latter is said to be 
related to O. elegantula Dalman, and is therefore referable to Prof. Hall’s subgenus 
« 
Dalmanella. 
Mus. Reg. No. 5543. 
Order THLOTREMATA, Beecher. 
Family RHYNCHONELLIDA, Gray. 
Genus RHYNCHOTREMA, Hall. 
1860. Rhynchotrema, HALL. ‘Thirteenth Report, New York State Cabinet of Natural History, p. 68, 
1883. Rhynchotrema, ee Ese ae Indica, ser. xiii, vol. i, p. 410. 
Rhynchonella 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 Brachiopoden, p. 396, 1860, Also Davidson's Mono, British Sil. Brach., p. 218, pl. XXvut, figs. 17-19, 
