310 



Mr. 0. Thomas. 



[Apr. 28, 



Whether it be heated rain-water, or heated sea-water containing 

 silica, the principle of the transmutation is the same. 



These siliceous beds are found, not only in the marine Silurian 

 (and possibly older) beds of tropical Australia, in which sponges are 

 comparatively rare, and in the Carboniferous rocks, but also in a 

 fresh-water deposit which caps a hill south of Mount Elder on the 

 Ord River, and about 500 feet above the level of the country, showing 

 that it must at one time have been the bed of a very extensive lake. 

 The upper beds are white limestone merging upwards as usual into 

 flint, calcedony, and green agates. These are 50 feet thick, and all 

 abound in a fossil, Planorbis, as determined by Professor McCoy, of 

 Melbourne University, who named it as a new species, Planorbis 

 Hardmani. His decision was confirmed by B. Etheridge, Junr., 

 and Dr. Woodward, and the specimens are at present in the Museum 

 at South Kensington. 



This rock is simply one mass of Planorbis shells all highly silicified. 

 I can hardly conceive that it was formed from sponge spicules, 

 especially as according to Ernst Haeckel (' History of Creation,' 

 p. 139) the main class of the Sponges lives in the sea, with the 

 single exception of the green fresh- water Sponge (Spongilla) . 



It is not probable then that these organisms would have existed in 

 these regions in sufficient numbers to form a rock 50 feet thick and 

 over two miles square at present. 



We have therefore examples at both ends of the scale in this one 

 country showing how improbable is the Sponge theory of chert. 



III. " On the Homologies and Succession of the Teeth in the 

 Dasyuridse, with an Attempt to trace the History of the 

 Evolution of Mammalian Teeth in general." By Oldfield 

 Thomas, British Museum (Natural History). Communi- 

 cated by Dr. Albert Gunther, F.B.S. Received April 4, 

 1887.- 



(Abstract.) 



The true homologies of the different teeth in the Marsupialia, and 

 especially in the Dasyuridm, have long been in a state of confusion, 

 largely owing to their perplexing superficial resemblances to the teeth 

 of the Carnivora and other Placentals, and to the incorrect homo- 

 logies thereon founded. This confusion has been chiefly in regard to 

 the premolars, of which some members of the family have two, others 

 three, while generalised Placentals have four, and it is therefore 

 necessary to prove which teeth have been successively lost in order 

 to find out the correct homologies of the remainder. 



