594 THE PALEONTOLOGY OF MINNESOTA. 



ICtenodonta socialls. 



directed posteriorly and situated about one-third of the length from the anterior 

 extremity; umbones carinate behind, the ridge having a distinctly concave outline 

 in a side view; posterior end of hinge projecting slightly beyond the ridge, so that 

 the post-dorsal region is not quite flat. Hinge plate comparatively short and weak, 

 widest posteriorly, very narrow beneath the beak and in front of same; denticles 

 small, seventeen or eighteen in each valve, in a continuous series, about eight of 

 them posterior and larger than the others. 



This neat and constant form is readily distinguished from C. nitida and C. medi- 

 alis, its nearest congeners, by the posteriorly carinate umbones, the less central 

 position of the beaks, its narrower posterior extremity and much weaker hinge plate. 

 The denticles also are less numerous. Casts of the interior of these three forms are 

 difficult to distinguish, but the task is not by any means hopeless when the speci- 

 mens are in a good state of preservation. The species is named for my colaborer on 

 the Gastropoda, Mr. W. H. Scofield, of Cannon Falls, Minnesota. 



Formation and locality. Aa entire example and nine valves were collected in the middle third 

 of the Trenton shales in the vicinity of Cannon Falls. Minnesota. A cast of the interior was 

 obtained at Minneapolis from the samq beds. The species has not been observed in the upper third 

 of the shales, but the overlying Galena shales have furnished a number of casts that I have not suc- 

 ceeded in distinguishing. These were collected at Cannon Falls and near Kenyon. 



CTENODONTA SOCIALIS, n. sp. 



PLATE XLII. FIGS. 59 and 60. 



Shell very small, moderately convex, transversely subovate, rarely exceeding 

 6 mm. in length, the average size about 3.8 mm. high by 5 mm. long; beaks small, 

 turning slightly toward the short posterior extremity behind which it is situated 

 between one-fourth and one-third of the entire length; umbonal ridge inconspicuous, 

 the convexity of the valves being relatively uniform. Surface almost smooth, no 

 markings save a few obscure concentric lines having been observed. Hinge plate 

 narrow, especially so under the beaks, widest posteriorly, comparatively long, arcuate, 

 the amount of curvature varying according to the length of the posterior end of the 

 shell, being greater when this part is shorter than usual; denticles small, nineteen 

 or twenty in each valve, six or seven of them posterior, several of the latter consid- 

 erably larger than any of the others. 



In a shell of this kind it is very difficult, if not impossible, to decide beyond the 

 possibility of error which end is the anterior and which the posterior. In this case 

 I have assumed that the short side is the posterior, because this end of the hinge 

 plate is the wider and bears the largest denticles, that being the prevailing condition 

 among species of this section. 



The small size, rather regularly ovate outline, moderate convexity and the 



