346 THE TANGANYIKA PROBLEM. 



Tanganyika form and that of the old Jurassic deposits. 

 Among the marine oolite there is a type of shell which is 

 closely similar to, but. quite distinct from, Purpurina bellona, 

 and which has received the name Purpurina inflata. 

 Hudleston, in his monograph on the Jurassic gastropods, 

 did not separate these two forms from one another, but he 

 figured the types, of which the two shells are characteristic, 

 upon different plates. It has long been a moot point 

 among conchologists whether Smith's genus, Paramelania, 

 was really distinct from Woodward's Nassopsis, and when 

 we compare the two forms of Purpiirina, to which I have 

 just referred, with the two living Tanganyika types, we 

 find that, not only is Purpurina bellona conchologically 

 indistinguishable from Paramelania damoni, but also that 

 Purpurina inflata corresponds equally closely with Nassopsis 

 nassa (see Figs, on p. 347, upper and lower). 



In the same marine Jurassic series of gastropods there is 

 another characteristic type of shell which has been grouped 

 together under the generic name Amberleya. The genus 

 was originally formed by Morris and Lycett, but was sub- 

 sequently modified by Hudleston, and, as amended by him, 

 the generic diagnosis runs as follows : — " Shell turbinate, 

 more rarely trochoid, rather thin, imperforate or nearly so ; 

 sub-elongate, frequently turreted ; sutural space wide ; 

 ornamented with spiral bands, usually spinulous or nodular, 

 some of which are prominent. The interspaces are finely 

 striated, the striae being slightly oblique to the axis. 

 Sometimes these fine lines are strong enough to represent 

 fine axial ribs. Base rounded, spirally ribbed and marked 

 by fine radial striae ; aperture sub-oval, but varying 

 according to age, in the adult more or less rounded, so as 

 to become sub-oval or sub-circular ; there is usually a con- 

 siderable deposit of callus ; outer lip thin, often crenulate." 



