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 suhtruncate 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. 



Seyeral 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 fillings and 

 M. recta (Modiolopsis recta Hall), from both of which M. rugosa diff'ers in the much 

 greater hight of the posterior end. The shape of the shell reminds one greatly of 

 I^chyrodonta and certain species of Cyrtodonta, hut 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 

 affinis. 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 arid locality. — Upper part of the middle third'of the Trenton shales, about six miles south 

 of Cannon Falls, Minnesota. 



Genus WHITELLA, Ulrich. 



Whitella, Uleich, 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 lina straight, from one-half to two-thirds the length 



