J. D. MACDONALD ON THE CLASSIFICATION OF THE GASTEROPODA. 7- 
is altogether ignored when all the air-breathing Gasteropods are associated in one so-called 
natural Order, JPulmonifera. 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 sufficient 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 Pulmonifera opercu- 
lata in the subjoined Table from their supposed alliance with Limax and Jle/Lr, and 
placing them in another division, with animals having an organization in more complete 
harmony with their own. The I* ulmonifer a inoperculata together with the Opisthot>ntn- 
* 
chiata (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 Patellida, which it resembles only in the shell, and the latter from the Apple-snails 
(Paladinida), 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 Siphonaria is a member of 
the Onchidiidce, while Amphibola, though prosobranchiate, exhibits an alliance with the 
Tectibranchs. However this may be, taking them in the order given, they render the 
passage from the Pulmonifera 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 altogether absent, while in the 
third it is strap-like. From 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 natural 
test of the affinities of unisexual genera with strap-like 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 initU it has been proved 
to be so. No scheme, for example, however plausible from its delusive applicability to a 
certain number of eases, can be accepted as natural, when conditions of greater value are 
violated by its adoption. The employment of the mere number of longitudinal rows of 
teeth in the lingual ribbon as a means of classification may be compared to the method 
of LinnaBus, who based his Botanical System on the number and aiTainjemeD t -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 
b7 its adoption. Hence it proves to be natural, sustained also by the axiom 
fticit saltumr 
natura non 
Errors with respect to sex may still lurk in the present Table, as I know to ha^e been 
*e ease 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 
