Prof. Owen on the Saurian nature of Placodus. 289 



respect of iraplautation, is of the ordinary crocodilian type in 

 respect of form, adapted to the prehension of fishes ; and there are 

 no palatal teeth. But the author remarks that such teeth exist in 

 the triassic Labjrinthodonts, with a disproportionate magnitude of 

 certain teeth which offers a certain analogy with the dentition of 

 Placodus. An account of the microscopic structure of the dentine, 

 enamel, and osseous tissue of the Placodus is then given. 



The extreme and peculiar modification of the teeth, in respect to 

 form and size, adapting them to the crushing and pounding of hard 

 substances, and the association of the Placodus with conchiferous 

 mollusks in such abundance as to have suggested the terms ' Muschel- 

 kalk,' ' Terebratuliten-kalk,' and the like, for the strata containing 

 them, concur in evincing the class whence the Placodi derived their 

 chief subsistence ; and the author points out the relation of a con- 

 stant disposition of the teeth, in all the known species, to the readier 

 cracking of shelly substance. A single row of teeth in the lower 

 jaw is always opposed to a double row in the upper one, playing, 

 with its strongest line of force, upon their interspace. Thus the 

 crushing force below presses upon a part between the two points of 

 resistance above, on the same princij)le on which a stick is broken 

 across the knee ; only here the fulcrum is at the intermediate point, 

 the moving powers at the two parts grasped by the hands. It is 

 obvious that a shell pressed between two opposite flat surfaces might 

 resist the strongest bite ; but, subjected to alternate points of press- 

 ure, its fracture is facilitated. 



Certain Australian lizards present teeth with large rounded obtuse 

 crowns, like those of certain Placodi, and have on that account 

 received the name of Cyclodus, for their genus. 



The author next proceeds to describe certain specimens of the 

 mandible or under jaw of the genus Placodus. The first of these he 

 refers to a species for which he proposes the name of Placodus 

 pachygnathus. The second may probably be the lower jaw of the 

 Placodus Andriani, Ag. ; but should it prove to belong to a different 

 species, the term bombidens would best express the specific pecu- 

 liarity in the shape of the grinding surface of the teeth. A third 

 species is named Placodus bathygnathus, in reference to the great 

 vertical extent of the mandibular ramus. 



All the above-described fossils are from the Muschelkalk member 

 of the triassic series, near Bayreuth, Germany, and have been 

 recently acquired for the Palfeontological Series in the British 

 Museum. 



June 17, 1858. — The Lord Wrottesley, President, in the Chair. 



" Description of some Remains of a Gigantic Land-Lizard {Mega- 

 lania prisea, Ow.) from Australia." By Prof. Richard Owen, F.R.S. 



The subject of this communication forms part of a collection of 

 fossil remains from Australia, recently acquired by the British Mu- 



