454: * THE PALEONTOLOGY OF MINNESOTA. 
[Platystrophia. 
These specimens are supposed to have been derived from the Galena limestone, 
and no others of the type of 0. subcequata occurring in this formation are known. 
Formation and locality.— Rsltq in the Galena liniestone(?) near Spring Valley, Minnesota. 
Collector. -K. H. V7inchell. 
Mus. Reg. No. 642. 
Subgenus PLATYSTROPHIA, King, 
1850. Platystrophia, King. Monograph of the Permian Fossils of England, p. 486. 
1892. Platystrophia, Hall. Palaiontology of New York, vol. viii, pt. i, p. 200. 
Description: "The name Platystrophia, proposed by Dr. King, has come into 
very general use for a group of orthids having a strikingly spiriferoid exterior. 
The hinge-line and area are long and straight and nearly equally developed on the 
two valves. Both are very convex, the brachial being the more so and bearing a 
very strong median fold corresponding to a deep sinus on the opposite valve. The 
valves are marked by sk-ong, sharp plications which extend over the fold and sinus, 
and the external surface is finely granulose, the latter feature being rarely well 
retained. This peculiar exterior, so unlike anything met with elsewhere in the 
genus Orthis, readily deceived earlier writers into referring the species to Delthtjris 
or Spirifer, and Mr. Davidson was the first to demonstrate* the true generic value of 
its internal and more essential characters. These are not materially different from 
those already described in the group of Orthis occidentalis. The delthyrium is open 
in both valves, being somewhat larger in the pedicle valve, and in old and gibbous 
shells of Orthis lijnx has often encroached to a considerable extent upon the umbonal 
region of the valve. The teeth are thick and very prominent, the muscular area 
comparatively small, but usually deeply excavated in the substance of the shell 
and not readily divisible into the component scars. In the brachial valve the car- 
dinal process is a simple linear ridge, always small and sometimes nearly obsolete. 
The dental sockets are comparatively small, the crural plates large and thick, 
uniting at their inner bases and produced into a prominent median ridge. The 
muscular area is quadruplicate and indistinct. The shell structure is very compact 
and finely fibrous, without punctation," 
Type: Terehratulites biforatus Schlotheim. 
"The genus appears in American faunas first in the Chazy and ranges upward 
into the Clinton and Niagara groups, attaining a great development in individuals 
and variety in external form in the Trenton-Hudson River fauna. It has also a 
considerable vertical range in the Silurian of Great Britian, Mr. Davidson citing it 
from the Caradoc, Upper and Lower Llandeilo and the Wenlock." (Hall, op. cit.) 
*Bull. Soc. Geol. de France, sec. ser., vol. x.\i, 1848, 
