196 Bulletin American Museum of Natural History. [Vol. XIV, 



would form the non-marine upper Jura. There are no inverte- 

 brate forms of decisive character in these beds to determine 

 between brackish and fresh water, but those found in bed 24 

 would incline toward fresh water. The change is clearly a 

 gradual one. These upper beds Marsh calls Atlantosaurus 

 beds, but as that genus has proved to be a synonym with Cama- 

 rosaurus, a less misleading term is desirable; and Scott ' has 

 introduced the name Como stage for exactly these beds. He 

 suggests that they belong to the Lower Cretaceous; and Logan 

 also uses the term Atlantosaurus beds as distinct from Jurassic. 

 There is no available invertebrate material in these upper beds 

 for comparisons; but the strikingly close relationships between 

 the British Purbeck mammalian fauna and the Wyoming mam- 

 mals cannot be left out of consideration. The considerable 

 number of British and Wyoming genera which are closely related 

 will compel us to consider the Como stage of nearly if not just 

 the same age "" as the Purbeck and retain the Como in the Jurassic 

 series. It is true there is no apparent unconformity between the 

 Como and Dakota, as would be expected if this Como stage were 

 dry land for a period as long as the Lower Cretaceous. 



During the Shirley period, the deposits on the Medicine and 

 Como anticlines were being laid down quite close to shore, as ap- 

 pears from the fact that within 30 miles to the south the Shirley 

 is unrepresented, and all of the deposits are strictly shallow water 

 sediments. However, during and especially at the beginning of 

 the Como stage, a considerable transgression took place (see 

 Knight's map) toward the south, removing the shore line to over 

 100 miles south. The deposits are also of shallow water. 



The Dinosaur remains could, however, have travelled such 

 considerable distances by floating. There could have been no 

 currents in the Como lake strong enough to transport gigantic 

 bones, for they were depositing clay. The bones are clearly 

 floated out to sea by the presence of considerable meat on them. 

 Most specimens must have started as complete carcasses, which 

 with decay of the flesh (or its consumption by crocodiles and 

 fish) have fallen apart, often making series of vertebrae, etc. 

 Doubtless such quarries as the Bone Cabin Quarry mark an eddy, 

 as there all sorts an d sizes of animals are collected together. 



' W. B. Scott, Introduction to Geology, p. 477. 



* Osborn, Jour. Acad. Nat. Sci. Phila, (2) IX, p. 187, 1888. 



