240 Dr. J. D. Macdonald 07i 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 Creseis and Hyalma are obviously carnivorous, they 

 are not on this account to be classed with the whelks or any 

 other carnivorous Gasteropoda 5 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 

 ConuSj Terehraj 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 Turritellidse and the Lamellariadse. 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 suffi- 

 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 camivora. Simple fanged teeth, or those without folia- 

 tions, are carnivorous, whether disposed in the manner of a 



