240 Dr. J. D. Macdonald on the Typical Value of 
account all other important structural particulars. Using a 
similar mode of reasoning to that adopted by Mr. Woodward, 
it might be said that although the dentition and habits of the 
pteropods Crese’s and Hyalea are obviously carnivorous, they 
are not on this account to be classed with the whelks or any 
other carnivorous Gasteropoda; nor, indeed, should they. But 
for this I will contend, that the dental characters are of equal 
importance in the discrimination of the Opossum from the 
Bandicoot, Clio from Pneumodermon, and of Murex from Bucci- 
num, and that the genuine gasteropodous families are to be 
distinguished by their teeth, subordinate to certain broader 
features of structure, including union or distinctness of the 
Sexes. 
The question as to the actual number of rows of teeth oc- 
curring in any particular genus or species ought not to super- 
sede the consideration of other characters afforded by them ; 
for it is just possible that the typical number forming part of 
the morphological plan of the family may be rendered obscure 
by suppression and modified development in minor types. I 
endeavoured on a former occasion to set forth this principle, 
and I have since found abundant proof of its correctness. 
Here, indeed, it may be assumed that there is a want of uni- 
formity; but, as even this appears to be amenable to fixed 
laws, the defect is more likely to be in our own philosophy 
than in the institutions of nature. The dental formula of 
Conus, Terebra, and Pleurotoma may be assumed to be a single 
series of fangs in each pleura, with a naked central space, 
characterizing the Toxifera of Dr. Gray; yet when in Clava- 
tula we find five rows of dental organs arranged as though the 
teeth of Mitra had been inserted between those of Bela or 
Mangelia, we recognize a primary and two minor types, de- 
pending upon the suppression of the central or the pleural 
teeth, as the case may be. Other examples of suppression of 
one or more of the members of the typical ribbon are to be 
found amongst the Turritellide and the Lamellariade. It 
is also of importance to observe the manner in which the 
dental processes are connected with the basal plates, and in 
particular whether they are recurved from the fore part of 
those plates or arise near or from their posterior border, in 
which latter case the teeth are not recurved, but point directly 
backwards. Thus the words recurved and direct would sufti- 
ciently express the two principal conditions here indicated, 
the dental points being in all cases retrorse. As a general 
rule, the teeth are recurved in the vegetable feeders, and direct 
in the carnivora. Simple fanged teeth, or those without folia- 
tions, are carnivorous, whether disposed in the manner of a 
