368 PETRIFACTIONS AND THEIR TEACHINGS. CHAP. IV. 



Essay, by Dr. Buckland, and the elaborate and highly philo- 

 sophical review of the subject, and the determination of all 

 the then known species, by Professor Owen, in 1839, in the 

 " Reports on British Fossil Reptiles," undertaken and pub- 

 lished under the auspices of the British Association of Science, 

 we are indebted for the present advanced state of this depart- 

 ment of British Palaeontology. The number of species of 

 both genera is now considerable, and many specimens, both 

 of Ichthyosauri and Plesiosauri, have been discovered in 

 various localities on the Continent. 



THE LIAS FORMATION. Although remains of Enaliosaurians 

 occur in all the formations from the Muschelkalk below the 

 Lias, to the Chalk inclusive, it is in the liassic deposits that 

 the greatest number, and the most perfect examples, have 

 been discovered ; and as the specimens which now engage 

 our attention were collected from those strata by Mr. Haw- 

 kins, I shall offer a few remarks on the geological characters 

 of the localities whence these splendid fossils were obtained. 



The Lias, situated between the Triassic, or New Red 

 Formation, and the Oolite, consists of a series of argillaceous 

 limestones, marls, and clays, which may be regarded as 

 forming the base of the latter formation, for there are scarcely 

 sufficient grounds for their separation ; the Upper Lias, and the 

 Inferor Oolite which lies upon it, having many fossils in com- 

 mon, and in some localities passing into each other. The 

 total thickness of the Lias varies from 500 to 1000 feet : the 

 strata have a very uniform lithological character, and con- 

 tain many peculiar organic remains. 1 



The Lias appears beneath the Oolite, through the south- 

 east of Somersetshire, and extends into Dorsetshire, forming 

 a range of cliffs, above four miles in length, along the sea- 

 shore at Lyme Regis, where it may be traced on the coast till 

 it gradually sinks beneath the Inferior Oolite. Lyme Regis 

 in Dorsetshire, and Watchet, Street, and Westbury, in Somer- 

 setshire, are the localities that have afforded the most in- 

 structive specimens. 



The subdivisions of the Lias are characterised by the 

 abundance of particular groups of fossils. Some of the strata 



1 See " Wonders of Geology," p. 521; and Sir C. Lyell's "Elements 

 of Geology," p. 273. 



