334- Geological Society. 



thinks, appertain to the Cetiosaurus, but he is of opinion that they 

 more probably belong to the Steneosaurus. 



In conclusion, it is stated that the vertebrae described in the paper 

 prove the existence of a saurian genus distinct from the Mega- 

 losaurus, Steneosaurus, Poikilopleuron, Plesiosaurus, or any other 

 large extinct reptile, remains of which have been discovered in the 

 oolitic series ; that the vertebra;, as well as the bones of the extre- 

 mities, prove its marine habits ; and that the surpassing bulk and 

 strength of the Cetiosaurus were probably assigned to it with car- 

 nivorous habits, that it might keep in check the Crocodilians and 

 Plesiosauri. 



5. " On the age of the Tertiary beds of the Tagus, with a Ca- 

 talogue of the Fossils," by James Smith, Esq., of Jordan Hill, 

 F.G.S. 



During a visit to Portugal in 1 840, Mr. Smith made a collection 

 of the organic remains in the tertiary deposits near Lisbon, for the 

 purpose of ascertaining their relative geological age. Since his re- 

 turn to England he has carefully examined the collection, assisted 

 by JVIr. George Sowerby, and ascertained that the series of beds 

 from which they were obtained belong to the miocene division of 

 the tertiary system, and to that portion of it which includes the 

 Bordeaux and Dax beds, rather than to any other yet described 

 deposit. He has, however, determined, by a careful comparison of 

 the Lisbon fossils with those given in the works of MM. de Basterot 

 and Grateloup, and with his own collection of Bordeaux organic 

 remains, that there is a greater difference than can be ascribed to 

 geographical distance alokue ; but he hesitates to assign to the Lis- 

 bon beds either a more ancient or a less ancient date. The propor- 

 tion of recent sheUs, he states, affords no assistance, as, according 

 to M. de Basterot, the existing species in the Bordeaux basin 

 equal 23 per cent., and according to M. Grateloup, 37 per cent., 

 whilst Mr. Smith's collection of Lisbon fossils gives 28 per cent. 

 The author is fully convinced of the soundness of the principle 

 of determining the comparative age of a tertiary deposit by the 

 proportion of recent species ; but he is of opinion, on account of 

 the great difficulty of defining species, that it is only possible to 

 arrive at an approximation sufficiently near to decide to which of 

 the great divisions of the tertiary system a set of beds may belong, 

 and not to the precise relative antiquity of two deposits of nearly 

 the same age. 



Prof. Agassiz has decided that several of the new species of Lis- 

 bon shells occur in the molasse of Switzerland, and he considers the 

 two series of strata as nearly contemporaneous. 



Mr. Smith refers to Mr. D. Sharpe's memoir on the neighbour- 

 hood of Lisbon *, for a description of the mineral structure of the 

 formation, confining his own remarks to pointing out the localities 

 and position in the series from which the fossils were obtained. 



• Proceedings, Geol. Soc, vol. iii. p. 28, 18.39 ; also Geol. Trans., Second 

 Series, vol. vi. p. 1. A list of tertiary shells is given in p. 113. 



