882 PKOF. G. C. BOURNE ON THE [Nov, 17, 



by reference to any other existing group. Thus the resemblance 

 of the shell of some members of the group to the Naticidse is due 

 to convergence. So also the possession of a single functional 

 etenidium on the left side, a character shared by the Trochidse 

 and Pectinibranchs, does not indicate that the Neritida^ stand 

 midway between these forms, but only that the right etenidium 

 has been suppressed independently in each. I have brought 

 forward evidence showing that the loss of the right etenidium is 

 comparatively recent in the Neritidse, probably as a consequence 

 of the great development of the accessory genital organs which 

 occupy all the space on the right side of the spacious mantle- 

 cavity. 



A consideration of the Helicinida?, the most specialized of the 

 Neritace*, must be postponed to the second part of this memoir. 



List of the principal Works referred to. 



1. Amaudrut, a. — La partie anterieure du tube digestif et la 



torsion chez les Mollusques gasteropodes. Ann. des 8ci. ISTat. 

 (8) vii. 1898, p. 1. 



2. Bergh, R. — Die Titiscanien, eine Familie dei- rhipidoglossen 



Gasteropoden. Morph. Jahrb. xvi. 1890, p. 1. 



3. Bernard, F. — Recherches sur les organes palleaux des Gas- 



teropodes prosobranches. Ann. des Sci. Kat. (7) ix. 1890, 

 p. 88. 



4. BiiOCHMANN, F. — Ueber die Entwicklung der Xeritina fliijvi- 



atilis Mull. Zeits. f. wiss. Zool. xxxvi. 1882, p. 125. 



5. Blundstone, E. R. — Glycogen in Vesicular Cells of Molluscs. 



Proc. Roy. Soc. Lond. xxxviii. 1885, p. 442. 



6. Boutan, L. — Sur le systeme nerveux de la Kerita polita. 



Comptes Rendus Acad. Sci. Paris, cxiv. 1892, p. 1133. 



7. Ibid. — Memoire sur le systeme nerveux de la Nerita polita et 



de la Navicella porcellana. Arch, de Zool. exper, et gen. (3) 

 i. 1893, p. 221. 



8. BouviER, E. L. — Le sy,steme nerveux et certains traits 



d'organisation des Neritidse et des Helicinida?. Bull. Soc. 

 Philomath, de Paris, (7) x. 1886, p. 93. 



9. Ibid . — Systeme nerveux, Morphologie generale et Classification 



des Gasteropodes prosobranches. Ann. des Sci. Nat. (7) iii. 

 1887. 



10. Ibid. — Le systeme nerveux des Neritides. Comptes Rendus 



Acad. Sci. Paris, cxiv. 1892, p. 1281. 



11. Brock, J. — Zur Neurologie der Prosobranchier. Zeits. f. 



wiss. Zool. xlviii. 1889, p. 67. 



12. Claparede, E. — Anatomic und Entwicklungsgeschichte der 



Neritina fluviatilis. Miiller's Arch, f . Anat. und Physiol. 

 1857, p. 109. 



13. Drummond, I. M. — The Development of Paludina vivijMra. 



Quart. Journ. Micr. Sci. xlvi. 1902, p. 97. 



14. Erlanger, R.von. — On the paired Nephridia of Prosobranchs. 



Quart. Journ. Micr. Sci. xxxiii. 1892, p. 587. 



