i 



margins. The truncation of the posterior end is a feature 

 which will at once distinguish it from that species. 



Geological Formation and locality. In the Trenton lime- 

 stone group (Buff limestone); at Beloit, Wisconsin. 



MODIOLOIISIS^ supbrbus, (n. s.) 



Description. Shell lar$;e eilongate, sub-elliptical in outline ; 

 cardinal line very slightly curving throughout its entire length, 

 and reaching no more than hal|f the distance from the beaks to 

 the posterior extremity of thfe shell. The posterior margin 

 from its junction with the hinge line, is but little curved till 

 near the extremity of the shell, where it is abruptly rounded to 

 the base line, which is gentlV arcuate throughout its entire 

 length except a slight sinuosity caused by a gentle depression 

 extending obliquely from the ^nterior side of the beaks to the 

 to the basal margin a litotle behind the beaks, anterior and nar- 

 row and abruptly rounded. Beaks moderately elevated, gently 

 incurved and approximnte, situated about one-sixth of the en- 

 tire length of the shell from the anterior end; a strong rounded 

 or sub-angular umbonal ridge extends from the beaks to the 

 postero-basal extremity of tlhe shell, becoming more gently 

 rounded as it recedes from th<e beaks. 



Surface marked by strong doncentric undulations, parallel to 

 the lines of growth. 



This is a large and line species, equalling the M. modiolaris 

 in size, but possessing nome peculiarities which makes the pro- 

 priety of its reference ito the genus doubtful. 



Geological Formation and Locality. In limestone of the age 

 of the Trenton limestone (Buff limeetone), Beloit, Wisconsin. 



GENUS AMBONYCHIA, Hall. 

 AMBONYCHIA cancellosa, (n. s.) 



Description. Shell obliquely sub-ovate in outline, with 

 ventricose valves, becoming compressed and attenuate towards 

 the extremity of the hinge line. Beaks slender, pointed and 

 directed forwards. Hinge line straight, a little less than the 

 greatest width of the shell. Anterior margin rather deeply 

 impressed in the upper part, and in some specimens showing a 

 shallow sinus, making what appears to have been a byssal 

 opening, the lower part is regularly rounded into the basal 

 margin. The posterior end is somewhat squarly truncate from 

 the extremity of the hinge line and gradually curving below. 



Surface marked by numerous strong concentric flattened 



