THE MOLLUSKS OF THE ROCKY MOUNTAINS. 45 



and, indeed, all over the Central Province. The finding of Papilla 

 Jilandi, heretofore known only as a fossil in Missouri River drift, 

 living and abundant, is an instance worthy of special mention. 



It would seem, then, that a range of high mountains, or any number 

 of ranges would not offer a serious obstacle to the migration of land 

 mollusks, or an insurmountable one to fresh-water forms. The wide- 

 spread dissemination of such slow-moving creatures is a curious argu- 

 ment for the length of time that the country must have remained in 

 substantially its present condition. 



The Sierras of which I have spoken are those which encircle Baker's 

 Park and the San Juan mining region, and extend westward to the 

 base of the great Uncompahgre Mountains, which trend northward not 

 far from the Utah line. This group of volcanic and quartzite peaks 

 constitutes the highest land anywhere in that region, and gives source 

 both to the Rio Grande del Norte and to the head-waters of the Great 

 Colorado River. Its steep southern sides are gashed with tremen- 

 dous gulches through which the Rio las Animas, the Rio La Plata, the 

 Rio los Mancos, and other streams, which go to make up the Rio San 

 Juan, flow out into the terrible canon-cut deserts that stretch away 

 across Arizona to the Gila River. For a few miles after emerging 

 from their rocky gates, these rivers water beautiful and fertile valleys, 

 which are cut through the sandstones upturned against the intruded 

 peaks, and which abound in springs. In these valleys are plenty of 

 timber and undergrowth, the climate is rarely cold enough for snow 

 even in winter, and there I expected to gather a rich conchological 

 harvest. In this I was not disappointed, only regretting that I could 

 not make a more thorough examination than was permitted by the 

 rapidity of our travel. Between the Animas and La Plata the trail 

 passes through a valley between the lowest of the foot-hills, where 

 there is a pond of several acres extent, resorted to by all sorts of wild- 

 fowl, inhabited by many forms of amphibious life, and choked with an 

 exuberant aquatic vegetation. Here were found thousands of limneas 

 of several species, and quantities of the common Planorbis trivolvis 

 showing a large range of variation among themselves. Like the lim- 

 neas, the planorbs w r ere extremely fragile in texture, which may be 

 owing partly to the soft bottom, and partly to the scarcity of lime in 

 the water ; and they were distinguished by a short vertical diameter, 

 which peculiarity, also, may have been acquired by them from the 

 necessities of their habitat, since snails having shells with small breadth 

 of beam could most advantageously pass between the stalks of stand- 

 ing water-plants that everywhere crowded the pond. But the as- 

 tonishing fact about this pond was, that on the shore were found per- 

 fect specimens although dead of the marine genus Truncatella^ a 

 broken specimen of an Area, and living crabs pronounced by Prof. 

 Sidney I. Smith, of New Haven, to be true salt-water forms belonging 

 to the family Astacidce, That these are survivors of the period, prob- 



