DREISSENSIA. 113 
to Septifer tegulatus, are regarded by him as probably identical with Sowerby’s 
species. I believe that view is probably correct, but without a larger series of 
specimens it is difficult to speak with confidence on the subject. The Aachen form 
is smaller and more variable in outline than the English species. I am indebted to 
Professor Holzapfel for the loan of several specimens. 
M. subarcuatus, Meek and Hayden, from the Fox Hills group, is considered by 
Meek? to be near M. lanecolatus, but is only known from an internal cast. 
Although it is possible for small fluviatile Lamellibranchs to be carried out to 
sea by currents and become entombed in marine deposits, such does not appear to 
have been the case with the species we are now considering, because (1) all the 
associated fossils are marine ; (2) the lithological evidence is not in favour of the 
strata having been deposited near a shore-line; (3) the species occurs at several 
horizons and in more or less widely separated localities ; and (4) some specimens 
have the two valves still united. 
[f, then, this species be truly marine, it might be urged that it is unlikely to 
belong to the genus Dreissensia, which at the present day is found in brackish and 
fresh waters only. That it does not belong to the genus Septifer is shown by the 
entire absence of radial sculpture, which characterises all the known species of that 
genus; in this feature, and also in the form of the shell, it agrees with living 
species of Dreissensia, differing only in the very young stages when (as shown by 
erowth-lines on adult specimens) the shell was rather more elongate pro- 
portionately. 
We have therefore apparently only two views to choose from concerning the 
generic position of the species here described as Dreissensia lanceolata :—(i) That 
it is an early marine form of Dreissensia ; or (ii) that, although possessing the 
characters of Dreissensia, yet it has no direct genetic connection with that genus, 
but is an instance of heterogenetic homceomorphy. If the latter view be taken, 
then this species must be regarded as the type of a new genus. The former view 
seems more likely to be correct, unless it can be shown that the Tertiary Dreissensix 
have no connection with this species, but have descended from some other generic 
type. 
That Dreissensia may have been marine at no very remote geological period 
seems possible from the fact that living forms occur in the brackish waters of the 
Aralo-Caspian area, etc.; that it probably was marine is supported by a study 
of its development,’ which differs from that of other fresh-water Lamellibranchs, 
and agrees closely with that of certain marine forms. It is further noteworthy 
1 “Tnvert. Cret. and Tert. Foss. Missouri” (‘U.S. Geol. Surv. Territ.,’ vol. ix, 1876), p. 69, 
pl. xxxviii, fig. 2. 
2 A detailed account of the development of Dreissensia polymorpha, with a full bibliography, has 
been recently given by J. Meisenheimer, ‘ Zeitschr. fiir wissensch. Zool.,’ vol. xix (1901), pp. 1—137, 
pls. i—xiii. 
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