838 EIGHTEENTH REPORT ON THE STATE CABINET. 



GENUS MODIOLOPSIS (Hall). 



MODIOLOPSIS DICT^US (n. s.). 



Shell broadly subovate, moderately convex, becoming somewhat gibbous 

 on the umbo ; beak about one-fifth distant from the anterior extremity ; 

 gradually expanding posteriorly so that half way between the beak and 

 the posterior margin it is once and a half as wide as in the line just 

 anterior to the beak. The straight hinge extends about half the length 

 of the shell, and the cardinal margin is thence gradually curved to the 

 posterior end. The cast of the hinge-line shows two narrow lateral 

 folds or teeth. Surface concentrically striated. 

 The length of a specimen measured is about one inch, with the greatest 



■width seven-eighths of an inch. 



The form of this species is very similar to Modiolopsis modiolaris of the 



Lower Silurian rocks ; but the hinge-line does not rise so abruptly on the 



posterior side of the beak, and the contraction below the beaks is not 



observed. 



Formation and locality. In the limestone at Racine, Wisconsin, and at 



Bridgeport, Illinois, Niagara group. 



MODIOLOPSIS RECTUS (n. s.). 



Shell elongate, narrow, moderately convex, beaks subanterior ; hinge-line 

 long and straight ; the greatest width of the shell is at the posterior end 

 of the cardinal line, narrowed equally above and below towards the pos- 

 terior extremity. 



The anterior muscular impressions large and strongly defined. The casts 

 show that there has been one strong subtriangular tooth beneath the 

 beak of the right valve, with one or two smaller ones, with correspond- 

 ing pits, in the opposite valves. The lateral teeth are very slender. 



The surface has been marked by concentric striae, and a few strong undu- 

 lations which are preserved in the casts. The length is about one inch 

 with the greatest breadth half an inch. 

 This species resembles Modioloysis {Tellinomya) machceraformis of the 



Clinton group of New-York, from which it difi'ers in having the beaks 



more nearly anterior and in being less narrowed posteriorly ; while the 



greatest width is at the posterior extremity of the hinge-line, instead of at 



-the beaks. 



Some specimens, which are scarcely specifically distinct from those 



described, have proportionally a somewhat greater width, but in other 



respects are identical. 



Formation and locality. In the limestone at Waukesha and Racine, 



Wisconsin, and at Bridgeport, Illinois : Niagara group. 

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