I 



The Saint Peter Sandstone. 73 



the anterior, which in turn curves abruptly at the antero-dorsal end. 

 Convexity moderate and uniform except for a low broad umbonal ridge 

 which extends from below the beaks towards the lower part of the poster- 

 ior margin. Point of greatest convexity just above the centre. One reni- 

 form anterior muscle scar near the anterior end, close to the pallia! line. 

 Pallial line simple, and marked off by a marginal thickening of the shell. 



Mopiolopsis subelUptica Ulrich, as described* is the nearest to this 

 species of any known. The difference in size, the position of the muscle 

 scars and the beaks, and the difference in outline and convexity are appar- 

 ent but the resemblance is great. The specimen is about medium size. 



South Saint Paul and Daytons Bluff. 



Modiolopsis affinis n. sp. 

 Plate n figure 9. 



Cast of medium size, length 15 mm, bredth 8 mm, beaks about one 

 fifth the distance from the anterior end, not prominent, but strongly curved. 

 The hinge line is curved and runs gradually into the truncate posterior. 

 The ventral margin is arcuate, and the anterior semi-circular. The con- 

 vexity is greatest anteriorly and along the strong umbonal ridge which 

 extends down and back to the extended end of the shell at the ventral pos- 

 terior margin. Above the umbonal ridge the shell is gently concave while 

 below and parallel with it the cast is flattened. The pallial line is not 

 distinct. 



The above description is of a single specimen which very much resem- 

 bles M. gregalis but is more convex in theantenor portion and has a strong 

 umbonal ridge and some indications that the shell was marked by coarse 

 elevated lines radiating from the umbonal ridge. The general outline of 

 the cast led to the expectation that specimens intermediate in form would 

 link this with M. gregalis n.sp. but although a large number of specimens of 

 the latter have been found, the distinctive characters still remain. 



South Saint Paul. 



Modiolopsis senecta n. sp. 

 Plate II, figure 17. 

 Shell large, length 33 mm, bredth 15 mm, convexity very moderate, 

 but slightly increased between the umbones and the posterior extremity. 

 Hinge straight and long. The ventral margin arcuate and it extends uni- 

 formly to the longitudinal extremities of the shell so that the anterior and 

 posterior margis are short obligue truncations. Pallial line and muscle 

 scars not visable. Beaks not projecting, low, and placed far anterior. 

 Only one specimen found. 

 South Saint Paul. 



Genus Tellinomya. 

 Neither of the two species referred to this genus have the character- 

 istic hinge teeth clearly preserved , but that is evidently because the sediment 

 or sand was tooicoarse to preserve a cast of such minute structure. 



Ext 19th Ann. Rep. Minn. Geol. Sur. March 3rd 1392 



