12 PALEONTOLOGY. 



either to foreign Lower Tertiary species or to living forms, are the grounds 

 upon which Dr. Engelmann and the writer, in 1860, referred these estuary- 

 beds to the Lower Tertiary. 



One of the species found in the Cretaceous sandstones at Coalville, 

 Utah, certainly agrees very closely with Corbula piriformis from the later 

 estuary beds at Bear River, as may be seen by comparing fig. 2, plate 14, 

 with fig. 2 a, plate 17. As the specimen from the sandstone, however, is a 

 mere cast, it is not possible to make a very satisfactory comparison. It is 

 also worthy of note here that there certainly is a species in the latest Cre- 

 taceous beds of California {Corhula alceformis of Gabb) that agrees very 

 nearly with our C. pyriformis from the Bear River estuary beds. Indeed, I 

 find very little in the figure or description of the California species to distin- 

 guish it from some specimens of G. pyriformis var. concentrica. Most of the 

 specimens of C. pyriformis are more coarsely and more irregularly ribbed 

 and furrowed than is shown in Mr. Gabb's figure; but the specimens vary 

 nmch in this character, some of them being nearly smooth, as in fig. 2 a, 

 plate 17, while others are regularly ribbed, as in fig. 2, or irregularly so, as 

 in fig. 2 a, and there are all conceivable intermediate gradations. As Mr. 

 Gabb, however, only figures one specimen, it is not possible to make an 

 entirely satisfactory comparison; though his species is most probably dis- 

 tinct from ours. 



This similarity of a few of the forms in the upper coal-bearing Creta- 

 ceous beds in Utah, AYyoming, and California, with species in the estuary- 

 beds in the Bear River country, and the general conform ability of 

 these formations, together with their association at the same localities, 

 and the non-conformability of the estuary-beds with the later Tertiary, 

 might suggest the inquiry, whether we ought not to carry up the line between 

 the Tertiary and Cretaceous here, so as to include the estuary-deposits in 

 the latter. 



This suggestion would certainly appear to receive some support, from 

 the fact that some of the vertebrate fossils collected by Dr. Hayden, appar- 

 ently from equivalent estuary-beds at the mouth of Judith River, on the 

 Upper Missouri, were regarded by Dr. Leidy as belonging to Cretaceous 

 types. Supposing that the change from marine to fresh- and brackish-water 



