132 KANSAS UNIVERSITY QUARTERLY. 



Locality: Freeze-out Hills, Wyoming; also reported from the 

 Black Hills. 



Remarks : The form above described resembles very closely the 

 northern European species, Avicula tenvicostata, but not the sj)ecies 

 Avicvla munsteri, as suggested by Meek.* 



Thf Athnitnsa 11 rns Fauna. 



Aside from the huge reptilian remains for which the Atlantasaurus 

 Beds are justly celebrated, it contains an interesting, although not 

 abundant, invertebrate fauna. The forms represented are all well- 

 known fresh-water types. Among them are species of Mollusca be- 

 longing to the genera Unio and Cyprena, and gastropods belonging 

 to the genera Valvata, Lioplacodes. and Viviparus. These inverte- 

 brates have been found in the Black Hills, in the Freeze-out Hills, 

 and in the Atlantasaurus Beds of southern Colorado. 



The Potomac formation of the Atlantic coast, which was deposited 

 under like conditions and has like lithological characteristics, also 

 contains a similar fauna. The habitat of the Potomac fauna, how- 

 ever, were probably waters of a brackish nature. 



The Wealden formation of England contains the greater part of 

 the genera which occur in the Atlantasaurus Beds, and is doubtless 

 of the same age. The two formations have similar lithological char- 

 acters, and four of the genera — Unio, Valvata, Planorhis, and Vivip- 

 arus — which are represented in the two formations by species having 

 practically the same degree of development, are not known from older 

 formations. 



Liojth(ro(7es refei'ims Meek. Plate XXXI, fig. 5. 



Shell small, planorbicular, composed of three or four whorls lying 

 in the same plane. The volutions are rounded, and increase rapidly 

 in size. The position of the decreasing volutions form concavities on 

 each side of the shell, the most jDrominent one being on the left side. 

 The surface of the shell is smooth and the aperture is nearly circular, 

 apparently, although the specimen is somewhat crushed. 



Dimensions: Diameter, maximum, 8 mm.; diameter of outer 

 whorl, 3 mm. 



Geological horizon : No. 22 of the section furnished a number of 

 these forms, while a few were collected from the clay of No. 21. 



Locality : The Freeze-out Hills, Wyoming ; also the Black Hills, 

 Dakota. 



Remarks: This specimen is much larger than that described by 

 Meek, but as there appears to be no other essential difference it is 

 probable that the latter is only a young individual. 



This genus was first recognized in the Wealden of England, and 



♦Geology of the Upper Missouri. 



