462 
large conical teeth, found in the Malton oolite, may, Mr. Owen 
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 vertebrz 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 vertebrze, 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, 
PGS 
During a visit to Portugal in 1840, 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 Mr. 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 alone; but he hesitates to assign to the Lis- 
bon beds either a more ancient or a less ancient date. The propor- 
tion of recent shells, 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, 1839; also Geol. Trans., Second 
Series, vol. vi. p. 1. A list of tertiary shells is given in p. 118. 
