LAKE TANGANYIKA — AN OLD JURASSIC SEA. 811 



tlieir shells could be sorted out into several groups which in 

 their extreme forms were quite distinct, but which were really 

 indissolubly connected together by innumerable transitional 

 types. Thus one type of Pyrgulifera agrees with Palu- 

 domus Pichleri (Hoern.) from the " Gosauformation/' 

 another with P. armatus (Math.) from the French chalk, a 

 third with P. lyra (Math.) from the same, a fourth with 

 Pyrgulifera humerosa (Meek) from the Laramie of North 

 America; while to a fifth and sixth Ajka variety there seem to be 

 no known corresponding forms. Since all these types are stated 

 by Tausch to run completely into one another, they can but be 

 regarded as connected polymorphs of one and the same generic 

 type, whatever the actual organisation of this genus may have 

 been. Tausch further points out that in Paludomus 

 Pichleri there are certain characters at the base of the 

 mouth which have led to this shell being described both as a 

 Paludomus and a Melanopsis. This melanopsid " mouth '^ 

 is not found, according to Tausch, in P. Stephanus (Bens), 

 but it is present in P. humerosa and in the Paramelania 

 Damoni of Tanganyika. Tausch therefore argues that the 

 ParamelaniaDamoni, Pyrgulifera humerosa, and those 

 forms of Paludomus which possess this peculiarity of 

 " mouth '^ are, together with certain forms of Melanopsis, 

 merely varieties of a single polymorphic type. This type 

 embraces also in its other modifications forms approximating 

 to Melania amarula, Lamarck^s type of the genus Melania. 

 I can fully confirm the observation of the remarkable similarity 

 of some of the Pyrguliferas collected by Dr. Oppenheim, 

 and now in the British Museum, to M. amarula; in fact, 

 some of these forms approximate far more closely to the 

 living M. amarula of Madagascar than they do to the Para- 

 melania of Tanganyika. Thus, whatever the dead Pyrguli- 

 fera may have been, its shells in their diflFerent modifications 

 agree with a great number of living types, and if it be really 

 legitimate to draw any conclusion from this complexity of 

 corresponding forms, it can only be said that Tausch^s work 

 has shown that there appears to have existed in the fresh water 



