OF CONCHOLOGY. 89 



NOTE ON THE GENUS ANISOTHYRIS, CONRAD, WITH A 

 DESCRIPTION OF A NEW SPECIES. 



BY W. H. DALL. 



The shells described by Mr. Conrad, Prof. Gabb and Mr. H. 

 Woodward, of which a large proportion come under the above 

 designation, are, from the laminated colored clays of the Amazon 

 Valley, identical, according to Prof. Orton, with the "Drift " of 

 Prof. Agassiz. They are from Pebas, near the mouth of the 

 Ambiyacu ; and Pichana on the south side of the Maranon, and 

 are also reported from other localities. While further informa- 

 tion in relation to their stratigraphical position would seem de- 

 sirable, from any point of view they form a very interesting 

 group. The authors above mentioned unite in regarding them 

 as mostly brackish water forms ; some of them undoubtedly are, 

 but on the whole I am inclined to the belief that the fauna was 

 more strictly marine than it has been regarded, and that the in- 

 termixture of fresh water and land shells may have been due to 

 their having been washed into the estuary in which the marine 

 forms were indigenous. This view receives some confirmation 

 from Mr. Woodward's remarks in reference to A. tenuis. Many 

 of the genera, judging from their recent representatives, are ex- 

 clusively marine, as far as we know, and there are none of them, 

 except Hemisinus and Bulimus, which may not have lived in 

 unadulterated sea water. Some of them are types of which we 

 have no recent representatives, and on these we can make no 

 decisive observations. The aspect of the shells, the well pre- 

 served colors, epidermis, and even remains of the cartilage, show 

 conclusively that they are of very late geological age, and I am 

 of the opinion that they cannot be older than Plioeene, and are 

 perhaps later. In this supposition I am supported by older and 

 much more experienced paleontologists. 



Genus CORBULA, Brug. 



Corbula, Brug., Enc. Meth. PL ccxxx, 1792. Lam., Hist. 

 An. s. Vert. v. p. 494, 1818. 



