1908.] ASPIDOBRAXCH GASTROPOD MOLLUSCS. 845 



behind the genital complex — some of its tubules are seen in 

 figs. 15 & 16, — and its duct issues from the anterior lobe, passes 

 ventrad of the receptaculum seminis, and opens into the light- 

 hand side of the mantle-cavity just below and to the right of the 

 oviduco-ccelomic funneL It will be observ^ed that the gland in 

 Septaria lies on the right side of the rectum, and cannot therefoie 

 be the exact homologue of the hypobranchial gland of the 

 Trochidfe, which lies to the left of the rectum, between the latter 

 and the left ctenidium to which it is related. It must rather be 

 regarded as the homologue of the right hypobranchial gland of 

 the dibranchiate Rhipidoglossa, and it seems probable that it 

 represents the additional gland of the right side described by 

 M. F. Woodward (41) in Pleurotomaria. The I'ight hypobi^anchial 

 (or additional hypobi'anchial) gland should be related to the right 

 ctenidium, but this has apparently disappeared in the Neritidfe.. 

 I cannot find any vestige of it in Septaria, but in various species 

 of Xerita and Neritina there is a small vascular organ projecting 

 into the mantle-cavity close to the aperture of the hypobi'anchial 

 mucous gland. This has been described in Neritina ffuinatilis 

 by Lenssen under the name of the " organe creux," and I shall 

 show in the latter half of this paper that partly on account of its 

 relation to the hyprobranchial gland there are good reasons for 

 regarding it as the vestige of the right ctenidium. 



Genera Nerita Adanson and Neritina Lamarck. 



It would be possible to give a detailed account of numerous 

 minute difierences in the alimentary tract, minor branches of the 

 nervous system, excretory organs, &c. in the various species of 

 JSferita, and Neritina that I have studied ; but the enumeration 

 of these details would be not only tedious but unprofitable. > I 

 have satisfied myself that in all essential features of the 

 anatomy of the alimentary tract, nervous system, respiratoiy arid 

 circulatory systems, and exci'etory organs, the various species 

 enumerated below are so similar to one another that they may be 

 included in a single description. Moreover, their main anatomical 

 features are so like those of Septaria that I may spare the reader 

 the trouble of perusing a mass of detail which Avould difter only 

 in unessential matters from what has ali-eadj' been desciibed in 

 the last-named genus. It is otherwise with the generative ducts, 

 especially the female ducts. I shall have to point out that the 

 specimens I have had the opportunity of examining fall into 

 three groups, defined by the constitution of the genital ducts. 

 The first group comprises the marine species unquestionably 

 belonging to the genus Nerita, as defined in conchological works. 

 The second group comprises the tropical species hitherto classed 

 in the genus Neritina and the genus Septaria. The third group 

 comprises the European Neritina fluviatilis, and to this must 

 probably be added the various European species of Neritina ; but 

 I cannot say anything definitely on this subject, for I have not 



