258 



iime stone deposit at Keokuck, Iowa, on the right bank of the 

 Mississippi. It resembled the teeth of the genus Psammodeus. 

 The specimen was worn on the posterior surface from the juxta- 

 position of another tooth. It was composed of very loose den- 

 tine, but at the base was a layer of more compact substance, 

 making it the type of a new genus. From its general resem- 

 blance to the teeth of the Port Jackson Shark, which, however, 

 are much smaller, an inference might be drawn as to the size of 

 the species. The Port Jackson Shark being three or four feet 

 in length, the size of this species must have been very great. 



There was a doubt, Mr. Desor remarked, as to the true geo- 

 logical position of the limestone from which the specimen was 

 procured. The Western strata cannot be so readily referred to 

 those of Europe as those of the eastern parts of the United 

 States. The distinctive traits of the strata of the East diminish 

 on going West. These strata mark the existence of a vast 

 ocean, the shore of which was along the Atlantic coast. He 

 saw, however, no reason to infer that the Western deposits were 

 deep sea deposits, the existence of fossil corals and Echinoderms 

 which live in water of moderate depth would indicate the con- 

 trary. He thought that many of the eastern deposits would be 

 reduced to one epoch. If at the present time we suppose an 

 elevation of the sea shore for a few feet, there would be a succes- 

 sion of strata of various composition similar to those referred to. 



Prof. Rogers said, that he had sometime since announced the 

 view that the New York and Pennsylvania beds of limestone 

 were of littoral origin, their outlines becoming confused and 

 blended on going west, until in the district of Cincinnati they 

 come in contact with the overlying clay, which in the east is 

 separated from it, making one common mid sea deposit of lime- 

 stone. This is shown by the diminishing size of the pebbles 

 and the thinning of the layers as they recede from the east. In 

 a similar way the palaeozoic horizons became confused towards 

 the west. The deposits which overwhelmed the animals of the 

 east did not reach the west, and they continued to live, so that 

 the inhabitants of different strata in the east occur together in 

 the west. Thus, where there was a layer of thick mud in the 

 east there is found in the west a deposit of thin clay, forming 

 shales interposed in the limestone. Prof. Rogers said, in con- 



