8o 
BULLETIN OF THE LABORATORIES 
GENUS ORTHIS, Dalman. 
Orth IS bifora'ta, var. lynx, Von. Buck. 
IV. forma reversata. 
{Plate XIII, Fig. 7.) 
Shell of the type known as var. lynx, especially those forms, which 
show a greater number of plications on the mesial fold and in the 
sinus. The name given to the form here described is expected to 
have only a local use to distinguish it from the variety lynx, as de- 
scribed and figured in the Ohio Geological Reports from the strata of 
the Cincinnati formations. Typical specimens of the different varie- 
ties of O. biforata from the I.ower Silurian formations of Ohio have 
an odd number of plications in the mesial sinus, and an even number 
on the mesial fold. Thus var. lynx has typically three in the sinus 
and four on the fold. When more than this number appear, the 
typical plications are the stronger. Typical specimens of O. biforata 
var. lynx in the Upper Silurian formations (Clinton Group), in the 
two forms known to me, have an even number of plications in the 
mesial sinus, and an odd number on the mesial fold. In the form re- 
versata the plications branch frequently, but the typical arrangement 
remains clearly defined. Although these distinctions are not expected 
to be of value elsewhere, they are too well defined and of too much 
interest in our local formations, not to be noted. 
Shell attaining a fair size, the dorsal valve a little larger; shell 
wider than long, with a sub-graduate outline ; no very gibbous forms 
have as yet been found ; hinge line less than the greatest breadth of 
the valves; cardinal extremities obtusely angular; lateral margins 
rather sinuous near the hinge line, rounding to the front, where it is 
somewhat sinuously rounded at the junction of the mesial sinus and 
fold. Beaks nearly equal, incurved and approximate, sometimes al- 
most touching ; cardinal areas nearly equal. 
Dorsal valve more convex than the ventral, its greatest convexity 
being near the middle. Mesial fold rather rounded, arising near the 
beak, becoming more prominent as it extends forward, with rounded 
sides; beak projecting beyond the hinge margin, strongly incurved; 
cardinal area directed backward, somewhat incurved. Foramen 
broad, triangular and not closed by the cardinal process. 
Ventral valve with a mesial sinus, beginning near the beak, ex- 
