238 Dr. J. D. Macdonald on the Typical Value of 



" The patterns or types of lingual dentition are on the whole 

 remarkably constant, but their systematic value is not uniform. 

 It must be remembered that the teeth are essentially epithelian 

 cells, and, like other superficial organs, liable to be modified 

 in accordance with the wants and habits of the creatures. The 

 instruments with which animals obtain their food are of all 

 others the most subject to those adaptive changes, and can 

 never form the basis of a philosophical system." And I add 

 here a note from the bottom of page 450 : — '^ The carnivorous 

 opossums have teeth adapted for eating flesh, but are not on 

 that account to be classed with placental carnivora." 



To state that the systematic value of the types of lingual 

 dentition is not uniform implies, first, that we are fully ac- 

 quainted with a subject which is yet avowedly only in its in- 

 fancy ; and, secondly, that from this knowledge notable in- 

 stances may be advanced demonstrating the truth of the asser- 

 tion. We are surely not to form a hasty conclusion to this 

 effect from the analysis of such a family as the Bullidse, for 

 example, including a mass of beings difiering as much inter se 

 as the families of Pteropoda, and much more than the three 

 acknowledged families of Heteropoda do. Now the principle 

 which I desire to maintain is that Gasteropoda whose general 

 anatomical characters, including the configuration, sculp- 

 turing, and minute structure of their shells, suggest their 

 distribution in the same group will be found still further to 

 be associated as well as distinguished by the type and pecu- 

 liarity of their lingual dentition — moreover, that prima facie 

 resemblance or difference, with or without conformity in the 

 dentition of the animals, may only require a little further 

 investigation to reveal their natural affinities or antipathies. 

 Anticipating myself a little, I think I may safely state, from 

 extended observation, that Concholepas^ Purpura^ Ricinula^ 

 Vitularia, or any other genus properly referable to the Mu- 

 ricidse will always be found with a lingual dentition unequi- 

 vocally on the type of that of Murex. But if some Pisanice^ 

 Ranella, Triton^ Fasciolaria, and other equally incongruous 

 genera are associated with them, it is no wonder that the den- 

 tition should be found to exhibit no " uniform systematic 

 value." For further illustration we may select the Buccinidae, 

 whose dental characters are so distinctive that any single 

 genus properly belonging to it can never be confounded with 

 Muricidse or any other family. The excellent authorities 

 Forbes and Hanley (vol. iii. p. 388), speaking of the genus 

 Nassa, remark that "it is one of the best marked and most 

 easily recognized groups, both as to shell and animal, among 

 the Muricid8e(!), thoi^gh some conchologists strangely persist 



