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40 EIGHTEENTH KEPORT ON THE STATE CABINET. 



and more approximate beaks, and less gibbous form, with a greater 

 ttatension in front and greater width behind ; which may prove specifically 

 distinct. 



* Formation and locality. In the limestone at Racine and "Wauwatosa, 

 Wisconsin, and at Bridgeport and another locality * (unknown) in Illinois* 



GENUS CYPRICARDITES (Conrad). 



CYPRICARDITES? QUADRILATERA ( n. s.). 



Shell somewhat quadrilateral, the angulated umbonal slope dividing the 

 valve into nearly equal areas. Valves inflated, height and depth nearly 

 equal ; beaks closely incurved and sharply angulated ; the angulation 

 gradually becoming obtuse, and continuing along the umbonal slope to 

 the postero-basal margin. A cast of the right valve shows a conspicuous 

 muscular prominence just anterior to the beak, with two strong lateral 

 folds marking the cardinal line : the entire surface is marked by low 

 concentric undulations. On each side and parallel with the umbonal slope 

 there is an extremely narrow groove and fold, indicating radiating striae 

 upon the surface of the shell. Just posterior to the beak there commences 

 a distinct groove, with a slight ridge on the lower side, which continues 

 about half way between the hinge-line and the umbonal angulation, but 

 not parallel with either, and extending to the posterior margin of the 

 shell. 



This species is angulated like many of the extreme forms of Myalina; 

 but the strong muscular marking, with lateral hinge-teeth, prohibit its refe- 

 rence to that genus. 



Formation and locality. In limestone at Bridgeport, Illinois : Niagara 

 group. 



Among the collections obtained from Wisconsin, there is a single shell 

 which is not referable to any established genus, though well preserved in 

 its general form and external characters. Although it would be very 

 desirable to know more of its structure before giving a generic name, I 

 shall nevertheless propose a designation. Its general form indicates that it 

 belongs to the Cardiacea, and this is confirmed by all that can be learned 

 of its structure. 



* Two specimens were received several years since from Prof. C. U. Shepard, which 

 were credited to Illinois, but the record of the particular locality had been lost. 



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