200 J. E. S. MOORE. 



In conclusion it may be interesting to reflect how all the 

 evidence which has been collected concerning the nature of the 

 halolimnic Gastropods invariably points to the vast antiquity 

 of these forms. First we have the wide dissimilarity of their 

 empty shells from those of any living types ; next their rigid 

 isolation to a solitary great lake^ which, judged from whatever 

 standard we may choose to adopt, is unquestionably of an 

 enormous age. Next we have the wonderful similarity of the 

 halolimnic shells now living in Tanganyika to those which 

 have been left fossilised at the bottom of the old Jurassic seas ; 

 and lastly, there are the morphological characters of the 

 halolimnic animals themselves, whereby they become mentally 

 depicted like nothing so much as the incompletely developed 

 embryos of numerous living oceanic types. 



EXPLANATION OF PLATES 20 and 21, 



Illustrating INlr. J. E. S. Moore's paper on '' The Molluscs of 

 the Great African Lakes/' 



Reference Letters. 

 a. Anus. g.g. Genital gland, g.a. Genital aperture. m.g. Mucous 

 gland. Int. Intestine, os. Osphradium. col. m. Columellar muscle, op. 

 Operculum, r.s. Radular sac. s.g. Salivary gland. B.m. Buccal mass. 

 cryst.s. Crystalline style, ce. (Esophagus, a.h.d. Aperture of hepatic duct. 

 r. Kidney. R. Renal aperture, b.s. Bi"ood-sac. g.a. Genital aperture. 

 st. Stomach, ft. Foot. e.g. Cerebral ganglion, pl.g- Pleural ganglion. 

 p.g. Pedal ganglion. I. com. Labial commissure, b.g. Buccal ganglion, ot. 

 Otocyst. sub. int. g. Subintestinal gangliou. sup. int. g. Supra-intestinal 

 ganglion, os.g. Osphradial ganglion. 



PLATE 20. 



EiG. 1. — Two views ot" a variety of the shell of Nassopsis. 

 Fig. 2. — Two views of another variety. 

 Fig. 3. — Embryonic shells and protoconch of Nassopsis. 

 Fig. 4. — Animal of Nassopsis removed from shell, showing the oper- 

 culum, op. 



