LAKE TANGANYIKA— AN OLD JURASSIC SEA. 315 



distinct types of Purpurina shell, one characterised by the 

 P. bellona (fig. 1a), the other by the P. in f lata given 

 in fig. 2a. Hudleston did not separate these forms as generi- 

 cally distinct, but figured the types of which they are charac- 

 teristic on separate plates. How closely similar this in f lata 

 type of Purpurina is to the living Nassopsis of Tanganyika 

 will at once be apparent from figs. 2 and 2a. The genus 

 Nassopsis was separated by Smith^ from Paramelania on 

 account of the difference in the operculum, but it is doubtful 

 if this distinction can be maintained from their anatomy ; in- 

 deed, I should be inclined to place Paramelania, Bytho- 

 ceras, and Nassopsis as species of one new family, the 

 Paramelanidae. The fact that there is more constant distinction 

 between the Tanganyika Paramelania and Nassopsis now 

 than that which used to exist between the bellona and 

 inflata types of Purpurina is just what we might expect, 

 since it is probable that these two forms would become less 

 transmutable as time went on.^ 



The Tanganyika Paramelania and Nassopsis are thus 

 identical with two forms occurring in the old Jurassic beds, and 

 the Paramelania corresponds more closely to the Bellona type 

 of Purpurina than it does to the Pyrgulifera of the chalk. 



In the same Jurassic series there is another characteristic 

 genus, Araberlya, which is specifically very variable in size, 

 sculpture, and in the character of its spines. Two forms are 

 represented in fig. 3a, the upper one from the collection in 

 the British Museum, the lower from Mr. Hudleston's collec- 

 tion. The history of this genus Amberlya is peculiar and 

 instructive, and will be found fully set forth in Hudleston's 

 monograph.^ The genus was originally founded by Morris 

 and Lycett, but was subsequently modified by Hudleston, and 



1 ' Proc. Zool. Soc.,' 1881, p. 559. 



2 There is a peculiarity in the base of the columella of some of the 

 Nassopsis shells which is not represented in those of the genus Purpurina, 

 but which is a permanent feature of the Jurassic Monodonta. So far as 

 Nassopsis goes this is an unimportant feature, since it is not constant in 

 the genus. 



' L.0C. cit., part 1, No. 6, pp. 274—279. 



