THE MOLLUSCS OF THE GEEAT AFRICAN LAKES. 169 



orgau given for T. rufofilosa (p. 158) would stand equally well 

 for that of S. zonata. In S. zonata, hovvever, owing to the 

 naticoid shape of the body, there is a forward displacement of 

 the internal viscera, which results in the pulmonary vein not 

 going directly forward to the base of the stenidium, as in T. 

 rufofilosa and most other Prosobranchs, but in its being bent 

 slightly backwards in a more or less acute curve before it reaches 

 the base of the gill (PI. 18, fig. 6). This relative displacement 

 of the heart and gill is fully described by Haller^ in the Naticas 

 Trochita radians, Ergaea plana, Crepidula peru- 

 viana (Lara.), Tanacus unguiformis (Lam.), and Crucibu- 

 lum (sp. ?), and it is curious to find that this displacement has 

 proceeded less in Spekia than in any of the forms which 

 Haller figured. 



The gill in S. z onata is very much like that of rufofilosa. 

 The leaves are similarly broad, low, and triangular, and their 

 apices are prolonged into the same littorinoid finger-like 

 processes (PI. 18, fig. 2). 



Such processes are fi^gured by Haller in the gills of Natica 

 lineata: they are present in the gills of Littorina (PI. 15, 

 fig. 1), but curiously enough they are not represented in 

 Haller's figure of the gill of Sigaretus neritoides 

 (Lam.). Neither are they present in the gills of the 

 Fauuus which I examined, and which are represented in 

 PI. 15, fig. 9. 



The osphradium is lodged in a groove beneath the gill; it 

 is simple, and slightly but distinctly tending to become pec- 

 tinated (PI. 18, fig. 2, OS.). At its outer extremity it is curiously 

 bent downwards and back, exactly repeating in this the con- 

 dition of the same organ in Sigaretus neritoides and 

 Natica lineata as represented by Haller (loc. cit., pi. xiii, 

 figs. 17—20). 



The nervous apparatus of S. z onata is the most interesting 

 feature which this form presents, the whole completely simu- 



1 Haller, "Die Morphologic der Piosobrancliia," iii, 'Morpb. Jahrb.,' 

 xviii, 1892, p. 451, and plates. 



