40 J* 



Royal Society :- 



errors in that particular would seem to be most likely to restore har- 

 mony and support the truth of the system. 



In the course of his inquiries the author was impressed with the 

 fact, that various genera of terrestrial Gasteropods, which agree with 

 each other as far as regards their respiratory organs and mode of 

 respiration, differ essentially in their general organization, whereas 

 they are in this respect severally related to fluviatile and marine 

 genera, which are obviously constructed on the same anatomical type. 



In this way a terrestrial genus, having few structural points of 

 agreement to connect it laterally, as it were, with others of the same 

 habit, forms a member of a beautifully connected natural series, 

 traceable from it through fluviatile and littoral forms to others which 

 are altogether marine. 



As an example of these relations the following Table is given, and 

 it might have been extended and rendered more complete, had the 

 author not preferred to limit it to such cases as have come under his 

 own examination. 



Sexes combined. 

 Otoconia. 

 Pavement. 



Sexes separate. 



Otoconia. 

 Lingual ribbon. 



Otolithes. 



Ribbon. 



Teeth nume- Rachis and Ilachis unise-Rachis unise- 

 rous, uniform Pleurae multi- rial, Pleuras rial, Pleurae 



in character. 



Terrestrial. 



fHelix. 



J 1 



>, Bulimus. 



rPhysa. 

 Fluviatile . . .< Planorbis. 



T J- 4. rScarabus. 



Leaxiing toj ^^^^^^^^^^ 



purely ma-^ Siphonaria. 

 rmegenera. ^i; j^.^^j 



serial. 

 Helicma. 



Navicella. 

 Neritina. 



Nerita. 

 Turbo. 

 Stomatella. 

 Broderipia. 



triserial. 



Cyclopliorus. 

 Diplommatin 

 Pupina. 



Cerithium. 



triserial. 

 I 



Melania. 



Hydrobia. 

 Planaxis. 

 Litiopa. 



This Table shows the natural affinities of four principal divisions of 

 terrestrial Gasteropods, proceeding, as it were, in parallel lines, with- 

 out any very obvious lateral connexions, through fluviatile and littoral 

 forms, conducting to certain marine genera distinguished by this 

 alliance from all others having no terrestrial representatives, and 

 behig therefore more restrictedly marine. It may be remarked that 

 the importance of the characters placed at the head of the Table has 

 been proved by the comparison of other anatomical particulars in 

 those genera, and so far their efliciency in other cases is sub- 

 stantiated. 



The author adds the following observations on the anatomy of the 

 Siphonaria and Amphibola, as bearing on their position in the first 

 series of the foregoing Table : — 



