Fossil Mollusks From the John Day Basin in Oregon 



these forms. Neither does it resemble except in gross characters 

 the United States Recent species H. occulta and H. orbiculata, and 

 the fossils H. ballista and H. tampae of Florida. It is one of the 

 most interesting Tertiary land shells thus far found, ranking with 

 Ammonitella, Boysia, Protoboysia, Helicostyla and Rhiostoma. 



Rhiostoma americana n. sp. 



Plate 1, figures 4a and 4& 



Shell composed of three whorls, the last increasing in size very 

 rapidly and projecting free of the spire. Spire elevated conic 

 with the whorls angulated acutely. Sutures deeply impressed. 

 A revolving lamella on the palatal wall of the last whorl continu- 

 ing and increasing in size to the aperture where it probably formed 

 a tooth. The columellar wall also had a tooth. The junction of 

 the columellar and basal walls acutely angular at the aperture. 

 Umbilicus apparently widely open. 



Measurements of type: greatest diameter, 8.50 mm.; least di- 

 ameter, 5.0 mm.; altitude parallel to axis 7.0 mm. (The measure- 

 ments of a complete shell would somewhat exceed these.) 



Only the type specimen has been found. It is from the John 

 Day Oligocene ( ? ) and is deposited in the University of Oregon. 

 (Type number 13). Unfortunately it is largely a cast and leaves 

 much to be learned of the shell. However the characters are so 

 unique that no hesitancy is felt in making it the type of a new 

 species. Its resemblance to the Siamese Rhiostoma Benson is 

 close and although the complete shell may show that it does not 

 belong in that genus it seems best to so refer it now. It is not 

 related to Boysia and Protoboysia, genera with Indian affinities 

 first added to our fauna in 1914.* 



The finding of this, apparently a land operculate with living 

 Asiatic relatives, in the United States Oligocene (?) is very remark- 

 able and furnishes a basis for much interesting speculation. The 

 groups of mollusks which then lived here are no less strange to 

 the conchologist than the mammals are to the mammalogist. 



"Cockerell, T. D. A., Land Shells from the Tertiary of Wyoming. Bull. 33 Amer. 

 Mus. Nat. Hist. p. 323, 1914. 



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