80 



terior (as shown in casts,) with strongly marked muscular 

 imprints ; the anterior one having its cardinal margin excavated 

 out of the hinge plate, not deeply seated, somewhat reniform; 

 the posterior imprint situated at near half its diameter below, 

 extremities of the lateral teeth irregularly oval; pallial impres- 

 sions usually distinctly marked, sometimes bounded by an 

 elevated ridge, in casts, showing a depression in the shell. 

 Lateral teeth situated obliquely to the hinge line, four in num- 

 ber, cardinal teeth several, the precise number not determined, 

 cavity of the beaks deep. External surface of shell unknown. 

 Length from one and a quarter inches to two inches, height 

 from summit of beaks to base greater than the length. 



This species differs conspicuously from all others except P. 

 rotundata in the highly elevated and nearly straight beaks; 

 and from that species in the proportionally greater distance 

 from the beaks to the base of the shell, and the less rounded 

 outline. The beaks of that species are much more curved 

 than in this. 



Geological Formation and Locality. Trenton limestone group, 

 (Buff limestone); at Janesville, Wisconsin, and near Dubuque, 

 Iowa. 



Collectors Br. Thomas Scott, T. J. Hale, James HaJJL 



GENUS MODIOLOPSIS, Hall. 



MODIOLOPSIS planus, (n.s.) 



Description. Shell, small compressed, the length a little 

 greater than the breadth, wider posteriorly. Cardinal line 

 Straight from the beaks to the posterior end, having a sr.b-alate 

 appearance; somewhat narrowly rounded anteriorly, the basal 

 margin straight, or but little curved, except at the anterior end. 

 Posterior end obliquely truncate. Beaks small, but little or 

 not at all incurved; umbonal slope moderately prominent, and 

 sub-angular. In the casts, the anterior muscular impression is 

 distinctly double, and well marked, the upper one situated 

 close to the cardinal border; the posterior impression is larger 

 and double, but less distinct, situated about two-thirds the dis- 

 tance from the beak3 to the posterior margin; pallial line 

 entire and somewhat distinctly marked. 



Surface marked by strong concentric lines of growth. 

 Length about three-fourths of an. inch. 



This species resembles the M. subspatulatus more than any 

 other species known to me, but is much more oblique, has a 

 proportionally longer hinge line, is more alate, and the um- 

 bonal slope continues to the junction of the basal and posterior 



