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. subwquata occurring in this formation are known. 



Formation and locality. Rare in the Galena limes tone(?) near Spring Valley, Minnesota. 

 Collector. N. H. Winchell. 

 Mus. Reg. No. 642. 



Subgenus PLATYSTROPHIA, King. 



1850. Platystrophia, KING. Monograph of the Permian Fossils of England, p. 486. 

 1892. Platystrophia, HALL. Paleontology 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 strong, 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 Delthyris 

 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 lynx has often encroached to a considerable extent upon the unrbonal 

 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: Terebratulites 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. Gc'ol. do France, sec. ser., vol. xxi. 1848, 



