1901.] NATURAL SCIENCES OF PHILADELPHIA. 515 



mens, some of which represented large and well-developed indi- 

 viduals. This species was therefore at least tolerably abundant 

 throughout the Jackson stage, and was probably in existence long 

 before. 



At Vicksburg there are two distinct horizons, as recognized by 

 Meyer,' but very inadequately and in part erroneously elucidated 

 by Hilgard. The lower Vicksburgian consists of alternate thin 

 strata of gray sands, sandy clays and variably, but usually loosely, 

 compacted white or gray limestone. The upper consists of a 

 much thinner bed of more or less red-brown marl, often indurated 

 into nodular masses, or subindurated, and without Irace of lime- 

 stone, having rarely, however, thin layers of glauconitic sands 

 and comminuted shells, in which entire specimens when found are 

 generally much distorted by pressure. 



The faunas of these two beds differ very markedly, and there 

 are probably not one-half of the species of either common to the 

 two. One of the chief points of distinction resides in the fact 

 that Orbitoides mantelli is virtually altogether wanting in the 

 lower or limestone bed and is abundant and fully developed in the 

 upper or marl bed. As this species existed in Jacksonian times, 

 however, it seems as though it must certainly occur in the lower 

 Vicksburg limestone, but at any rate it is so rare that I have 

 never observ^ed a specimen. The incongruity, therefore, of call- 

 ing the Vicksburg limestone an Orbitoidal limestone is sufficiently 

 evident; possibly the error occuri'ed by reason of the washing 

 down into the ravines of some material from the upper marls. It 

 is consequently certain, from the facts above mentioned, that 

 Orbitoides mantelli and its varieties existed through a considerable 

 portion of the entire duration of our early middle Tertiary, inclu- 

 ding and subsequent to the Jackson stage, and that it became alter- 

 nately abundant or semi-extinct according 'as surrounding condi- 

 tions favored or retarded its development. 



In regard to Pecten poidsoni, it is only necessary to refer to the 

 report on the Coastal Plain of Alabama, by Dr. E. A. Smith, where, 

 on page 237, this species will be found listed with the Bashi fossils 

 of the Lignitic stage. As there is no more reason to doubt the 

 correctness of this identification than there is to doubt the identity 



^ The two lower horizons of Meyer constitute, in ray opinion, but one. 



