DREISSEXSIA. 113 



to Sepfifer tegtdatns, 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. siiharcaatas, Meek and Hayden, from the Fox Hills group, is considered by 

 Meek^ to be near M. lanccolatus, 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. 



If, 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 'orackish and 

 fresh waters only. That it does not belong to the genus Septlfer 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 

 growth-lines on adult specimens) the shell was rather more elongate pro- 

 portionately. 



We have therefore apparently only tAVO 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 homoeomorphy. 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 Dreissensias 

 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 "Invert. Cret. and Tert. Foss. Missouri" ('U.S. Geol. Surv. Territ.,' vol. ix, 1876), p. 69, 

 pi. xxxviii, fig. 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. Ixix (1901), pp. 1 — 137, 



pis. i — xiii. 



15 



