J. D. MACDONALD ON THE CLASSIFICATION OF THE GASTEROPODA. 76 



is altogether ignored when all the air-breathing Gasteropods are associated in one so-called 

 natural Order, Fulmonifera. Indeed the difference is so great in the two principal divi- 

 sions of this Order, that the most superficial of all distinctions, namely, the presence or 

 absence of an operculum, is sufiicient to distinguish them. In this way, bisexual animals 

 with a pavimental dentition are associated with unisexual animals having a lingual rib- 

 bon, and several more striking external differences. Indeed the characters common to 

 both are such as apply to Gasteropods generally, affording no proof of their morphological 

 agreement. This, therefore, must be my apology for separating the Fulmonifera opercu- 

 lata in the subjoined Table from their supposed alliance with Lhnax and Helix, and 

 placing them in another division, with animals having an organization in more complete 

 harmony with their own. The Fulmonifera inoperculata together with the Opisthobran- 

 cJiiata (M. Edwards) are retained in the position which Mr. Woodward gives them, as two 

 natural series demanding no special change. I have, however, been obliged to place be- 

 tween them the transitional genera Siphonaria and Amphibola, removing the former from 

 the PatellidcB, which it resembles only in the shell, and the latter from the Apple-snails 

 {Paludinidce), with which, even taking into account the obscure resemblance of its shell, 

 it cannot have the remotest affinity. I refrain from the use of a family term to include 

 those genera, because I have much reason to believe that Sipihonarla is a member of 

 the Onchidiidcs, while Amphibola, though prosobrancliiate, exhibits an alliance with the 

 Tectibranchs. However this may be, taking them in the order given, they render the 

 passage from the Fulmonifera inoperculata to the Opisthobranchiate families easy and 

 natural. 



The members of the second division in the Table are unisexual, and in this primary 

 character differ from those of the first. They admit of arrangement into three sections, 

 in one of which the dentition is pavimental ; in another altcjgether absent, while in the 

 third it is strap-like. Prom actual observation and comparison of genera belonging to all 

 the families cited, with one or two trifling exceptions, I am convinced that, first, the 

 character, and, next, the number of the dental plates and processes, afford a truly natiiral 

 test of the affinities of unisexual genera wdth strap-lUvc dentition. 



In studies of this kind I always bear in mind a grand principle, for which I am in- 

 debted to Mr. W. S. Macleay, namely, that no character is natural until it has been proved 

 to be so. No scheme, for example, however plausible from its delusive applicability to a 

 certain number of cases, can be accepted as natural, when conditions of greater value are 

 violated by its adoption. The employment of the mere number of longituchnal rows of 

 teeth in the lingual ribbon as a means of classification may be compared to the method 

 of Linnaeus, who based his Botanical System on the number and arrangement of the 

 stamens, and would threaten at first sight to be equally artificial ; but I find that the legi- 

 timate sway of characters of greater as well as of minor value is not interrupted or violated 

 by its adoption. Hence it proves to be natiu-al, sustained also by the axiom, " natura non 

 facit saltum." 



Errors with respect to sex may still lurk in the present Table, as I know to have been 

 the case in a former one. I must state, however, that the sexual character of those 

 instances which I have not been able to determine personally is supported by good 



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