108 On the Tertiary Shells of the Amazons Valley. 



certain that the genus, represented by half a dozen species and 

 nearly a thousand specimens, must decide the point. 



In its living analogue, the genus Azara ox Potamomya^ Sby., 

 we have just the evidence Ave need to argue upon. 



^^ Azara lahiata,^''sivjs Darwin," lives buried in the mud of the 

 Rio de la Plata, hut not above Buenos Ayres, and consequently 

 in water which is little influenced by the superficial ebb of the 

 river. . . . The same species is found widely dispersed in banks 

 (fossil) over the Pampas near S. Pedro and many other places 

 in the Argentine Republic nearly one hundred English feet 

 above the Rio Parana."* 



Here, theii, we have the most complete analogous conditions 

 established between the Pampas formation and the Amazonian 

 shell-clay. In both, the shells have died, as they lived, in 

 banks by hundreds and thousands, all with their valves united 

 in jwirs and closed. 



Can any one doubt for a moment that which Mr. Hauxwell's 

 discovery clearly proves — namely, that the estuary of the 

 Amazons was once in long. 72° west, lat. 3° south, or more 

 than 2000 miles above its present position? Indeed, as Sir 

 Charles Lyell has well observed, there is nothing new in these 

 phenomena ; they are but " the natural result of the oscilla- 

 tions in the level of the land, extending over large continental 

 areas, by which the fall of rivers is lessened at certain periods, 

 giving rise to accumulations of matter more or less lacustrine, 

 while subsequently, when a movement in the opposite direc- 

 tion takes place, the rivers cut through their old deposits, re- 

 excavating the valleys and often eroding them below their 

 original depth "f. 



I cannot close this notice without adding that my best thanks 

 are due to Dr. Baird for assisting me in the determination of 

 these curious and interesting shells. 



P.S. Since the foregoing was written, I have received from 

 Mr. Robert Damon, of Weymouth, a second series of Amazo- 

 nian Tertiary shells, forwarded to him by Prof. Orton ; they, 

 however, contain no new forms in addition to the series sent 

 home to Mr. Janson by Mr. Hauxwell himself, save two spe- 

 cies of the genus Iscea, namely Iscea (Mesalia) Orfoni, Gabb, 

 and Iscea tricarinata, Conrad. Prof. Orton thinks that Istea 

 tricarhiata is possibly the young oi Iscea Ortoni] we do not, 

 however, see any evidence of carime on the whorls of the latter. 

 It is highly probable that there are two species placed together 

 under /. Ortoni — one in which the spire is short and the 



* Geological Obsen^ationa on Coral Reefs, &c. vol. ii. chap. i. pp. 2 & 78. 

 t ' Principles/ chap. xix. p. 468. 



