214 



Natural History Bulletin. 



very closely; while Zonites limatiiltcs, Zonites fulvns, and Val- 

 lonia fulchcJhi average rather larger in size than recent shells. 

 The remaining species are very similar to their modern repre- 

 sentatives, or like the boreal species heretofore mentioned, do 

 not now occur in the region occupied by the Loess. That 

 the variation in the comparative sizes of the recent and fossil 

 shells is not more uniform is not at all rem*arkable when we 

 consider the great variation in adaptability to circumstances. 

 Most of the species represented in the Loess have a very wide 

 distribution at present, and a study of sets of recent shells 

 from various altitudes and latitudes reveals the fact that the 

 species which are now the most constant (some of which are 

 also the most widely distributed), as well as those which seem 

 now to be the most variable in form and size possessed the 

 same characteristics during Lcess times. 



A NEW SPECIES OF FRESH WATER MOLLUSK. 



B. SHIMEK. 



Ancylus obliquus, nov. sp. 



PLATE III., FIGS. 5a 5b y: . 



Shell elevated, thin, transparent, horn -colored, with a 

 yellowish-brown epidermis; aperture ovate, conspicuously 

 wider anteriorly, in many (especially young) specimens 

 slightly reniform b}' a barely perceptible incurving of the 

 right margin, the anterior, left and posterior margins reg- 

 ularly rounded, the right slightly incurved, straight, or but 

 slightly convex; apex somewhat acute, elevated, strongly de- 

 flected posteriorly and to the right, and curved downw^ard, in 

 most specimens quite overhanging the posterior right margin 

 of the shell; the apical portion of the shell (one-half or more) 

 is strongly laterally, or rather obliquely, compressed, a char- 



