364 Natural History Bulletin. 



be of interest. From San Carlos to the mouth of the Los 

 Sabalos River, a distance of over 20 miles, the river valley is 

 broad and low, in most places swampy, and the river is com- 

 parativel}^ sluggish — in fact it is here but an estuary of the 

 lake. That the lake itself once extended much further east 

 seems certain. The fact, already noted, that the Rio Frio is 

 now crowding the lake shore westward indicates this, and an 

 additional bit of evidence was obtained b}^ the writer on the 

 last trip from Castillo to San Carlos. This is a shell-bank on 

 the cattle-ranch of Manuel Vargas on the right bank of the 

 San Juan River about 12 miles below San Carlos. The river 

 here flows almost due east, and has cut away a portion of the 

 shell-bearing stratum, thus forming an almost vertical bank 

 which, in the portion under consideration, varies from 12 to 

 15 feet in height, the general surface gradually rising from 

 this toward the south and south-west. For a distance of 

 about TOO yards this bank is literally packed from top to bot- 

 tom with the shells of two small species of Unio (species not 

 yet determined), many of them being cemented together and 

 imbedded in calcareous tufa. The shell-deposit extends south- 

 ward from the river-bank for an unknown distance; it was 

 traced back by the writer for at least 50 yards, but as he was 

 not provided with the necessar}"^ implements for making exca- 

 vations, the limit could not be ascertained. Many of the 

 shells were found in their natural vertical position, and many 

 more were still united by the ligament, all this, coupled with 

 the fact that the species are now living in abundance in both 

 the river and lake, indicating that the shells were deposited in 

 situ. The facts noted above indicate that the deposit was 

 formed by ordinary natural methods, but it is only fair to state 

 that sharp chips of flint, rude and probably merely accidental 

 fragments, are associated with these shells." However no 

 other evidence (if this may be considered evidence) that the 

 deposit was formed by other than ordinary means was found, 

 and until stronger evidence is secured we must look upon it 

 not as a shcIl-mound but an ordinary geological stratum. It 



