368 MEXICAN BOUNDARY SHELLS— BALL. volxix. 



iiortliern IVIexico is well known, will it be possible to generalize correctly 

 ^ on the geojirapliical distribution of tlu' niollusks of this region and the 

 extent to whi(;h ditl'erent elements are represented in the fauna of the 

 boundary. 



For a long time peculiai'ities in the distribution of the land mollusks 

 of this and adjacent regions have suggested to tlie writer that during 

 the Oligocene the higldands of northern ^Mexico were separated from the 

 mainland of Xorth America by an arm of the sea. It is certain that 

 the peninsula of Lower California was so separated, and the separation 

 may have persisted until even more recent times. The existeiu'e of 

 comparatively fresh marine sliells, fossil in the sands of the desert far 

 to the eastward of the Colorado, and the fact that the height of land 

 or watershed between the Ccdorado and the l\io Grande is at present 

 only 0,000 or 4,000 feet above the sea, while the evidences of volcanic 

 activity are abundant — all these facts give color to the h^'pothesis, 

 which requires fuller and more exact investigation for its establish- 

 ment. If such a barrier has not been iuteri)osed, it is difficult to ac- 

 count for the failure of the laud-shell fauna of western Texas to make 

 a distinct impression on that of the boundary region, and to find 

 reasons why the genus Epiphranmophora should have failed to extend 

 its range to the eastward of the Mexican uplift. But this problem is 

 for the future to solve, and we may rest content with having stated it, 



FRESH-WATEB SPECIES. 



Genus LIMN^^A, Lamarck. 



LIMNiEA BULIMOIDES, Lea. 



Found sY)aringly at Fort Worth, Texas, and in the Eio Grande near 

 El Paso, Texas. 



LIMNiEA DESIDIOSA, Say. 



A few very slender specimens Avith elongated spires were found in 

 the drift of the Santa Cruz IJiver, Tucson, Arizona, by Dr. Mearns. 

 Also at San Eafael, Xew Mexico; Ashmund. 



Genus PHYSA, Draparnaud. 



PHYSA MEXICANA, Philippi. 



Collected by Dr. Mearns at Fort Worth and Fort Clark, Texas; in 

 the Guadalupe Canyon, and San Bernardino Biver, New Mexico; at 

 Lake Balomas, in the Mimbres Valley, Mexico; from Seven Wells, the 

 Colorado Biver, and tlie Santa Cruz River near Tucson, Arizona; at 

 Laguna, 20 miles north of Catui)o, and at Cameron's ranch, San Diego 

 County, California. Some strongly shouldered specimens in a subfos- 

 sil state, from the Colorado desert, are perhaps a variety of this species, 

 which is extremelv variable. 



