COLOMBIA AND VENEZUELA 



355 



been usual to separate off the two latter countries as forming a dis- 

 tinct "I'eruvian" sub-region; but there is, as will Ije seen, aljsolutely 

 no line to be drawn between the Mollusca of Peru and those of 

 Ecuador ; nor would one, on geographical considerations, expect 

 to l)e al)le to draw such a line. A better method of sul)division, 

 so far as the species of the whole eastern ])ortion of the region 

 are concerned, would be to group the Mollusca according to the 

 altitude at which they occur, were it not that the evidence on 

 this point is at jn-esent but fragmentary. We know, however, 

 that all ahjug the line of the Andes certain species, more parti- 

 cularl}' of Bidiiaulvs, occupy tlieir own i;ones of elevation, some 

 ascending as high as 10,000 feet above the sea, and never occur- 

 ring on the plains. 



In the northern portions of this sul)-region, ("entral American 

 and West Indian inftuence is felt to a certain extent. Thus there 



Fig. 233.— a, Orthalkv.s 

 Dchunjhiae Keeve, 



Ecuador ; B, Jiulimvs 

 ( Pachyot us) eijrcijius 

 Jay, Brazil. 



are eight Glandina and one Sfrc2)tosfijl(( in \'enezuela and Colombia 

 togetlu'r witli one or two speci(>s of Cistala, Ghondropoma, rro- 

 serpiiia, and Cyli ml reiki , while a single Strophia (decidedly a 

 .straggler) occurs at Cura^vao. In Demerara and Cayenne there 

 are three or four species of DciitrUnrui. lu Ecuador, however, 

 (llandiua dimiiushes to three s])ecies, and in Peru disappears 

 altogether, althougli one Htirptustyla occur.s. Similarly the West 

 Indian operculates are reduced to one C'/iundroj'uma (Ecuador), 

 and disappear entirely in Peru. 



