X46 Carl O. Dunbar, 



Genotype: A. nelsoni, n. sp. 



The general form of this type of shell resembles on the one 

 hand Holopea, and on the other Diaphorostoma and Platyceras, 

 but from these associated genera it is readily distinguished by its 

 laterally compressed whorls, and especially the deeply channeled 

 aperture. 



Etymology: av\6<s, channel, -f- ottt], aperture. 



Aulopea nelsoni, n. sp. 

 Plate IV, figs. 14-16 



Description: Shell low-spired, of about three gradually 

 expanding volutions. Whorls laterally compressed, almost twice 

 as high as wide, rather narrowly rounded below and above, 

 widest a little above the middle. The outer side of the whorl is 

 marked by a broad, slightly concave zone that is more clearly 

 defined above than below. The inner whorls rise only a short 

 distance above the outer. Suture rather deep. Base with a 

 deep but moderately narrow umbilicus. Aperture narrowly 

 ovate, rounded above, and having a deep, rounded channel at the 

 base. Inner lip thin and somewhat wrinkled where it bounds the 

 umbilicus, and very thin but entire where in contact with the 

 preceding whorl. Surface marked by fine lines of growth, and 

 more distant, uneven, and coarser growth varices. The growth- 

 lines are curved so as to be gently concave forward on the upper 

 side of the whorl and broadly convex forward on the outer side. 

 A distinct grooved line running along the outer edge of the base 

 marks the outer side of the anterior channel. At this line the 

 growth-lines are sharply deflected backward and then cross the 

 base with an even curvature, swinging forwai^d again as they 

 enter the umbilicus. These lines indicate that the channel in the 

 aperture was about 5 mm. deep. 



Dimensions: Height, 30 mm.; width, 2/ mm.; height of aper- 

 ture, 22 mm. ; width of same, 13 mm. 



Occurrence: Rockhouse shale at Rockhouse on Horse Creek, 

 Hardin County. 



Name: This gastropod is named after the state geologist of 

 Tennessee, Mr. Wilbur A. Nelson, 



