202 PALAEONTOLOGY OF NEW YORK. 



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 

 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 cardinal 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. 



Plattstrophia is represented by a series of forms, all commonly regarded as 

 referable to Schlotheim's Terebratulites biforatus, or as presenting only varietal 

 differences from this species. The genus appears in American fiunas 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 Britain, Mr. Davidson citing it from the Caradoc, Upper and Lower 

 Llandeilo and the Wenlock.f 



HETERORTHIS 



(nora. propos). 



VIII. Group of Orthis Clytie, Hall. 



PLATE Vb, figs. 20-24. 



1861. Orthis, Hall. Fourteenth Rept. N. Y. State Cab. Nat. Hist., p. 90. 



1862. Orthis, Hall. FiftecMith Kept. N. Y. State Cab. Nat. Hist., pi. ii, fig. 4, 5. 

 1875. Orthis, Miller. Cincinnati Quart. Jour. Sci., vol. ii, p. 34. 



187.'). Orthis, Hall and Whitfield. Geology of Ohio; Pateontology, vol. ii, ]i. 75, pi. i, figs. IS, 19. 



The species Orthis Clytie, from the Trenton horizon at Frankfort, Kentucky, 

 is the sole representative of a peculiar strophomenoid or leptaenoid exterior 

 accompanied by internal characters which are distinctly orthoid, but quite pe- 



* Bull, de la Soc. Geol. de Fi-ance, vol. xxi, 2d ser., 1848. 

 t Davidson, Briti.sh Silurian Brachiopoda, p. 272. 



