292 GEOLOGICAL SURVEY OF THE TERRITORIES. 



* The oldest beds in the Bear Biver country of Utah and Wyoming, 

 properly belonging to the tertiary, and, as above stated, so intimately 

 related to the latest cretaceous, contain species of Corbula, Cyrena, (Gor- 

 bieula), perhaps Ostrea and a univalve related to Melampus, directly asso- 

 ciated with several species of Goniobasis, two of Unio, one or two of Me- 

 lantho, several species of Vivipanis, one of Tiara, &c, showing clearly 

 that these strata were deposited in brackish waters.f These shells also 

 exist in great numbers, and are preserved in a condition showing that 

 they could not have been transported far by currents, but that they must 

 have lived and died at least near where we now find them. 



All paleontologists are aware of the fact that the remains of fresh and 

 brackish-water shells do not generally present such well-marked peculi- 

 arities of form, ornamentation, &c, in beds of different ages, as we see 

 in marine types, so that they cannot always be relied upon with the 

 same degree of confidence in identifying strata that we place in marine 

 forms ; some of those from oldest cretaceous being, for instance, very 

 similar to existing species. So far as I have been able to compare the 

 species from this formation with described forms from other parts of the 

 world, they generally agree most nearly with lower eocene types ; the 

 Corbiculas and Tiara being very similar to forms found in the lower 

 lignites of the Paris basin, and at the mouth of the Ehone in France. At 

 the same time it is worthy of note, that most of these shells are quite 

 unlike any of the known existing North American species, and one of 

 them (Tiara humerosa) belongs even to a genus entirely unknown among 

 the existing Melanians of the American continent, though found inhab- 

 iting the streams of Madagascar, the Fejee Islands, &c. One of the 

 Uniones (IT. belliplicatus) resembles in its ornamentation some of the 

 South American species and the genus Castalia, much more nearly than 

 it does any of the recent North American species, although having the 

 form and hinge of a true Unio ; .and another abundant bivalve, found in 

 the same associtiaon (Corbula Anisothyris pyriformis) seems to be allied 

 in some respects to a peculiar group recently described from a pliocene 

 or miocene formation on the Upper Amazon of South America, by Mr. 

 Gabb, under the name Pachydon, and afterward renamed Anisothyris by 

 Mr. Conrad, because the name PacJiyodon had been previously used for 

 another genus. 



This last-mentioned shell (Gorbula pyriformis) was referred by me, pro- 

 visionally, to Azara, because it occurs in the sams beds with fresh and 

 brackish-water forms, and has the general aspect of some species of Cor- 

 bula, which group is believed by some good authorities on conchology 

 to include Azara as a sub-genus; while none of our specimens showed 

 the hinge. Among some of the latter collections, however, I found speci- 

 mens by which I was enabled with considerable difficulty to succeed in 

 working out the hinge, and found that it does not agree with that of Azara, 

 but apparently conforms almost exactly with that of Corbula, with prob- 

 ably the exception of some regular furrows on the tooth of the right 

 valve. From its brackish- water habits, however, and its general simi- 

 larity of form to Anisothyris or (Pachydon) erectus, Conrad, I referred it, in 

 manuscript, to Anisothyris, placing that group as a sub-genus under 

 Corbula. Soon after I mentioned, in a letter to my friend Mr. Conrad, 

 that I had discovered that this shell does not possess the hinge of Azara, 

 and that I had referred it to Anisothyris. 



* These remarks from this paragraph to the end of the first paragraph ending on 

 page 295, inclusive, were extracted and printed in pamphlet form, excepting the foot 

 notes, on the 18th February, 1871. 



t The conditions, however, might have been such that the saltncss of these estuaries 

 was, at first, very little diluted by the streams that brought from the adjacent shores 

 tho fresh-water types. 



