THE MOl-LUSCS OF THE GREAT AFRICAN LAKES. 197 



anterior stomachic diverticula which contain crystalline styles, 

 similar to those of the genera S trombus, Pteroceras, Ros- 

 tellaria, Murex, and Trochus; and it has already been 

 shown in a former paper ^ that there is no reason whatever to 

 doubt that these structures, when they appear in the Proso- 

 branchiata, are like the heart when perforated by the rectum, 

 as in the Trochidse, to be considered as the retention of a con- 

 dition common to the ancestors of both Prosobranchs and 

 Lamellibranchs alike. 



The presence of these similar arrangements in so many of the 

 halolimnic Gastropods, which in all other ways are so widely 

 separated from each other, is thus only intelligible if we 

 suppose that peculiar stomachic apparatus was once universal 

 among the ancestors of the Prosobranchiata of the present 

 day, and that in the remnant of an old fauna, such as that 

 now existing in Tanganyika, we encounter a more abundant 

 representation of an archaic state of things.^ 



The presence of a well-developed mucous gland in Nas- 

 sopsis, the extremely simple character of the gills and the 

 osphradium, the simple reproductive apparatus, are features 

 which all further dissociate Nassopsis not only from the 

 hitherto known fresh-water forms, but also from the more 

 modified Tsenioglossa at present existing in the sea. 



All the preceding structural peculiarities which demonstrate 

 the archaic character of Nassopsis are, lastly, fully substan- 

 tiated by the curious condition of the nerves. The great rela- 

 tive length of the cerebro- and pleuro-pedal connectives suggests 

 the condition found in Haller^s so-called '^ longi-commissurate 

 forms," such as Strom bus, Pteroceras, and their allies; 

 while in the presence of a labial commissure we are confronted 



^ 'Quart. Jouru. Micr. Sol.,' loc. cit., pp. 198, 210. 



2 In the case of Nassopsis we can go, however, further than this ; for com- 

 paring tlie spiral csecum in the stomach of this genus with the well-known 

 and similar structure in the stomachs of the Rhipidoglossa, the two are found 

 to be apparently identical, and the retention of this typically primitive 

 feature in a form which also in other ways is so distinctly archi-tsenioglossate 

 is a morphological fact of the highest interest. 



