159 



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 Palaeontological Series in the British 

 Museum. 



The Memoir is accompanied by numerous drawings. 



March 18, 1858. 

 The LORD WROTTESLEY, President, in the Chair. 



The following communications were read : 



I. " On the probable Origin of some Magnesian Rocks." By 

 T. STERRY HUNT, Esq., of the Geological Survey of Canada. 

 Communicated by THOMAS GRAHAM, Esq., Master of the 

 Mint. Received July 10, 1857. 



The deposits of mineral matters from natural waters oifer many 

 points of interest to the geologist. Besides the rock-salt and gypsum, 

 which in many cases have doubtless been formed by the spontaneous 

 evaporation of saline waters, it is well known that many mineral 

 springs charged with carbonic acid under pressure, deposit great 

 quantities of earthy salts when they come to the surface, and that 

 the travertines thus formed often constitute extensive masses. The 



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