68 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



and without any theoretical preconception, the 

 gradual variations of the shells of the group Cancel- 

 laria cancellata, from the Miocene type of the Vienna 

 Basin down to the living form in the Mediterranean. 

 But the most striking genealogical series of all 

 is furnished to us by the researches of Neumayr 

 and Paul on the Paludina of the Levantine fresh 

 water strata in the Danube Basin. The Paludines 

 or Vivipara are fresh-water molluscs which dwell 

 in large numbers in our lakes and rivers ; their 

 shells have the form of somewhat lengthened 

 spirals with convex whorls devoid of all ornament. 

 In the Pliocene lacustrine strata there are found, 

 at the bottom of the series, Paludines with smooth 

 whorls similar to the existing types ; when found in 

 rather younger strata, the whorls of the spiral of the 

 Paludines become flattened, then hollow out with a 

 flattened median line, with a tendency to a carina 

 [keel] becoming more and more marked at the top of 

 each coil ; then a second carina appears at the base of 

 the spiral whorl ; finally each of these carinas becomes 

 denticulated and bristles with increasingly distinct 

 tubercules in the higher strata of the formation. 

 It is a strange and almost unexplained fact that 

 similar tendencies to the formation of keels and 

 tubercules manifest themselves in the same Levan- 

 tine stage, among other fresh-water molluscs be- 

 longing to very different families the Melanopses, 

 the Neritince, and the Unios, for example. But 

 whatever may be the cause, we can establish among 

 these various kinds of molluscs continuous genea- 

 logical series the evidence of which forces itself upon 

 any observer. It is very curious to note that almost 



