236 BELOGONA-SIPHONADENIA. 



merely the ancestors of groups now living in Europe, and lateral 

 branches of those phyla. The presence of snails belonging not only 

 to modern genera, but to modern subgeneric or sectional groups, as 

 low as the lower Miocene, indicates that for the roots of even these 

 weakly characterized divisions, we must look still earlier; and the 

 large spaces of Eocene time can scarcely be held sufficient for the 

 differentiation of the genera now occupying the European tract. 

 The absence of Belogona Siphonadenia from all regions except those 

 now occupied by that group is negative evidence tending toward 

 the view that the group developed its special peculiarities in that 

 quarter of the world; and while this sort of evidence is always in- 

 conclusive, it has some weight in the total absence of facts making 

 against it. A provisional hypothesis might be outlined, holding 

 that the primitive Belogona (with the genital structure like Helico- 

 stylci) spread westward before or at the beginning of Eocene time, and 

 in the Eur- African tract the stock became modified by the removal 

 of the mucus glands from the dart sack, and their change into the 

 tubular form, into the siphonadenious type ; subsequently splitting 

 into a considerable number of genera. Those genera which have 

 spread again from this center are mainly minute forms capable of 

 living in cold regions, such as Vallonia and Acanthinula; but the 

 presence of Helicodonta and Metodontia in China, and of the East 

 Asian genus Eulota in Europe, indicates a more southern connection 

 also. These exchanges between the faunas of the east and west ex- 

 tremes of the Palrearctic continent are remarkably few, however ; 

 and we are compelled to believe that since the incursion which 

 brought Belogona and many other Oriental types to Europe, the 

 climatic or other conditions prevailing in Central Asia and Siberia 

 have been unfavorable to the spread of land mollusks. 



Of course there is no reason to believe that Helices of the Epiphal- 

 logonous type did not also reach Europe with or before the Belogona ; 

 and they may have survived there during Eocene and even Miocene 

 times ; in fact the genera Dentellocaracolus, Fridolinia,etc., may re- 

 present such survivors. But to state that this is the case, or that 

 those genera belong to the Epiphallogona (i. e. are related to Cara- 

 colus, Obba, Chloritis, etc.) is merely to state one's pleasingly sensa- 

 tional flights of fancy as scientific truth. The evidence showing the 

 presence of Ephiphallogona in Europe at any time, rests now upon 

 the finding of certain rather heavy, rudely sculptured forms; but 

 they are neither heavier nor more coarsely wrinkled than some Hemi- 



