748 NOTES ON NORTH AMERICAN SHELLS STEARNS. 



As Avoiild iiatiii'all\' he .supposed, of a Ibrui iiiliabitiiig so <ireat an 

 area, cousideiabh' variation is cxliibited, and one iinds adults in sonio 

 places with small shells, in others with shells conspicuously large; 

 some with elevated and some with depressed shells. Again, in some 

 localities, the growth lines are delicate, and the shells also liglit and 

 thin; others have heavy shells, and a coarse sculpture. Another and 

 more striking varietal character is the occasional presence of a tooth- 

 like prominence on the parietal Mall, and sometimes a toothlike process 

 is seen at the base of colunudla on the peristome. 



The genus Mesodon is rei)resented on the Pacific coast of North 

 America by several species. At the present time there is a great gaj) 

 between the western and northern extension or limit of the group as 

 we trace it wexticard from the Atlantic side of the continent, and the 

 extremest eastern locality, at which it has been found as we follow it 

 eastward from the Pacific coast, liegarding, as 1 do, both H. Toivn- 

 sendiana and H. ptychojyhora as Mesodons, and considering the latter as 

 a variety of the former, we find these West or Pacific-coast forms ex- 

 tending eastward as far as Idaho, where j;/^67a>7>//o/Y/ has been detected, 

 near Salmon Eiver and in the valleys and on the slopes of the Bitter 

 Eoot Mountains; it also occurs in Montana, according to Binney. Be- 

 tween western Idaho and Minnesota there is, it will be seen, a great 

 gaj), in Avhich we have no evidence of the existence or presence of any 

 form of Mesodon. It is not, however, unreasonable to suppose, that 

 sooner or later this long reach will be materially shortened by the de- 

 tection of Mesodon at new localities, both in the easterly and westerly 

 margins of the present boundaries. 



From the Miocene of the John Day region,* in the neighborhood 

 designated as the North Fork of the John Day liiver, Oregon, longi- 

 tude 1190 40'^ latitude 44° 50', as given by Prof. Condon, we find 

 Mesodon associat<Ml with i/. [AriouUi) fidelis, H. [PaUda) perspeetiva 

 and the rare aiul curious Ammonitella Yatesii of Dr. Cooper.t To the 

 Mesodon, which 1 regarded as an undescribed form, I gave the name 

 of Dallii. The other species, from the John Day beds, are familiar to 

 the coneetoi- and student of recent land shells, though Y<(tesii is about 

 as rare an Ji del is is common. 



Mesodon IhtUii differs from any of tlie living representatives of the 

 group iidiabiting the Pacific States. It suggests an ancestral form, 

 from which may have proceeded the species known as eolnmhiania, 

 devia, germana, etc. Ammonitella Yatesii is so exceedingly rare, and 



* Bulletin of the IT. S. Geologioal Survey No. 18. On the Marine Eocene, Fresh- 

 water Miofene and other Fossil Mollnsea of Western North America, by Chas. A. 

 White, M. 1). Washington. 1885. 



tThis species is generally referred toby authors as (ionosUnim Yulcniu but Cooper's 

 genus AmmonitcUa, 18B8, whicli is based on this form, is valid and should therefore 

 stand, as Raliues<ine's (ioiiofitoma (applied to a grou)) of lishes), 1810, has ])reccdence 

 over the use of sail! name in the Mollnsca, (Hehl.. ls:{7) by twenty-seven years, as 

 well as over Pfeilfer's use ol' (iuuoxtomu in 187J). 



