564 THE PALEONTOLOGY OF MINNESOTA. 
{ Whitella. 
umbonal ridge. Anterior end descending abruptly from the beaks, below rounding 
sharply into the nearly straight ventral border; posterior margin produced and 
strongly rounded in the lower half, obliquely subtruncate above, forming an obtuse 
angle at the junction with the hinge line; the latter very gently arched. Surface 
marked with strong concentric wrinkles and finer lines of growth. Shell substance 
of moderate thickness. 
Hinge plate strong, flat, slightly arcuate, the upper half of the width posterior 
to the beaks, finely striated lengthwise. Cardinal teeth small, situated just beneath 
the beaks, directed toward the postero-basal margin, with one in the right valve and 
on each side of it a deep socket for the reception of the two teeth of the left valve. 
Anterior muscular scar rather distinct, subcircular, situated immediately beneath 
the teeth. 
Several additional specimens of this well marked species were collected during 
the summer of 1892, among them an entire left valve showing the hinge. This has 
two cardinal teeth and no posterior laterals, so that there can be no longer any 
question as to the generic position of the shell. Only two other species belonging 
to this genus are known to have been described. These are M. tenera Billings and 
M. recta (Modiolopsis recta Hall), from both of which M. rugosa differs in the much 
greater hight of the posterior end. The shape of the shell reminds one greatly of 
Ischyrodonta and certain species of Cyrtodonta, but in the former the cardinal teeth 
are much stronger and the ligament internal instead of external. The hinge of 
Cyrtodonta, with its posterior lateral teeth and curved, more numerous, and longer 
cardinal teeth, is quite different, but when the interior is hidden the collector may 
experience some trouble in distinguishing the species from the associated Cyrtodonta 
affimis, Still; there is one difference that will serve his purpose very well, namely, 
the anterior end of the latter is rounded and somewhat produced beyond the beaks, 
whereas it descends abruptly from the beaks in the Matheria. 
Formation and locality.—Upper part of the middle third of the Trenton shales, about six miles south 
of Cannon Falls, Minnesota. 
Genus WHITELLA, Ulrich. 
Whitella, ULRICH, 1890, Amer Geol., vol. vi, p. 176. 
Shell thin, obliquely quadrangular or suboval, equivalve, inequilateral, more or 
less ventricose. Umbones very prominent, the beaks strongly incurved; umbonal 
ridge prominent, subangular or sharply rounded. Cardinal margin straight or 
slightly convex, the edges inflected to form a sharply defined escutcheon extending 
beyond the beaks sometimes quite to the anterior extremity of the shell; area finely 
striated longitudinally. Hinge line straight, from one-half to two-thirds the length 
