SMITH ON THE AGE OF THE TERTIARY BEDS OF THE TAGUS. 411 



science of conchology we can only expect to arrive at an approximation 

 sufficient perhaps to determine to which of the great divisions of the 

 system a tertiary deposit helongs, but not, as in the present case, to 

 throw any light on the relative antiquity of two formations of nearly 

 the same age. 



Professor Agassiz recognized several of the new species as belonging 

 to the Molasse of Switzerland, and is inclined to consider the deposits 

 as being nearly contemporaneous. 



I am indebted to Mr. G. B. Sowerby for the descriptions of the 

 new species annexed to the catalogue, and to his son for the figures ; 

 the initials G. B. S., with or without a mark of interrogation, indicate 

 his opinion as to the identity of the shells to which they are attached, 

 with their recent or fossil analogues. 



The mineral structure of the strata in which they are imbedded has 

 been so fully and so accurately described by Mr. Sharpe, in his com- 

 munications to the Geological Society*, as to render any details on 

 my part superfluous ; I shall therefore only notice them to point out 

 the localities and the place in the series from which the fossils were 

 collected. 



The upper beds are composed of loose sand and gravel, and are 

 known as the Golden Sands of the Tagus, from the gold which has 

 occasionally been extracted from them. They are destitute of organic 

 remains, but below them we find a series of beds of yellow sand and 

 calcareous sandstone and blue marl, rich in marine remains. Mr. 

 Sharpe has termed them the Almada beds, from the town of that 

 name on the south side of the Tagus opposite Lisbon, where they are 

 most fully developed, and it was from them that the fossils here 

 described were collected. The number of species is upwards of 150, 

 but many of them are mere casts, or m too imperfect a state to allow 

 their specific characters to be made out. Those which have been 

 determined amount to 1 24 species ; of which 20 are new and peculiar 

 to the deposit, 5 1 occur in the Bordeaux beds, 1 7 in the Faluns of 

 Touraine, 1 5 in the Sub-Apennine and Sicilian beds, 8 in the eocene 

 of the London and Paris basins, and 35 are recent. Several of them 

 also occur in the deposits of Vienna, Switzerland, Turin and the 

 Morea. 



LTpon the whole, I am inclined to consider this deposit as belonging 

 to the older miocene ; that is, that it is older than the Touraine beds, 

 and nearly of the same age as those of Bordeaux and Dax. 



JDentalium entails. Recent. Fossil at Bordeaux, and in all the 

 divisions of the tertiary system. 



— — Radula, Brocchi, p. 262? Nearly straight, with 9 or 10 slender 

 granulated strise : this is probably the same as that mentioned 

 by Brocchi "a, strie granulose." Fossil in Italy. 



Serpida vermicularis. Recent. 



fascicidaris, Lamk. Recent. 



* Proceedings of the Geological Society, vol. iii. p. 29. 

 VOL. III. PART I. 2 F 



