TUE TERTIARY GEOLOGY OF THE 
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clue to the exact determination of the age of the deposits in wliich they occur. It is 
true that an examination of the liomotaxial deposits of Europe shows the genera 
Phohuhmya and Panopea to be more especially characteristic of the lower or even 
lowermost liorizon, of the Eocene series, as in the English and French basins, but no 
special inference can be drawn from this circumstance, since the species are not the 
same, and the genera survived through the succeeding periods to the present day. 
In the case of Odrea comjireasirodra, however, we have a much more tangible, point. 
The species, first described and figured by Say (.7 ournal of the Academy of Natural 
Sciences, iv, p. 1 o3), is certainly very intimately related to the Odrea BeUovwma of 
Lamarck, and apparently undistinguishable from certain varieties of that species.* 
Now this sjiecies, although not exclusively restricted to the lowest Eocene beds, is 
nevertheless highly characteristic of the Thanet sands, below the London Clay proper 
and also Iwlow what was formerly designated as the “Plastic Clay” series, where it 
constitutes a true basement accumulation ; and it holds almost precisely the same 
n'lation to the beds of the Paris basin, where, according to Deshayes i^Auimnux s. 
Verthhrta, Basdn <1e Paris, ii, p. 117), it occupies the horizon of the Bracheux sands, 
'file specie.s, wherever found, appears to be considerably restricted in its vertical range, 
and its occurrence, therefore, in some of the American deposits would seem lo afford 
some more decided indiration of the true age of those deposits than could be obtained 
from the characti'r of the limited number of its contained fossils taken as a whole. 
.\sso( iate<l with Odrea compresdrostra were found casts of the large Gvcallma gajantea 
(C onrad, Journ. Acad. Nat. Sciences, vi, p. 215, 1830), a species which appears not to 
Ik‘ rc-preseuted in any of the equivalent European formations. But in Virginia, in 
Ix-ds which can be .shown to be the direct equivalents of those of Maryland, there 
occurs, in ad.lition to the C. gigantea of Conrad, a second species of CucuUcea' Wve C. 
oaochda of Rogers (Trans. Am. Philos. Soc., new ser., vi, p. 373; Latiarca idonea, 
(our., Proc. .\cad. Nat. Sci., 1872, p. 53— no locality stated), which, if not identical 
with the C. crassa/ina or Lamarck, from the Bracheux sands of the Paris basin, is 
C( rtnin y mo.st intimately related to it, and can be considered in every way as its 
ro,,re8ont„.ive.t It sl.onl.l also be stated tl,at the only other species of 
