594 THE PALEONTOLOGY OF MINNESOTA. 
(Ctenodonta socialis. 
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.—An 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 same 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, ”. 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 oyate outline, moderate convexity and the 
