the Lingual Dentition in the Gasteropoda. 239 



in mingling it with Buccinum^ The buccinoid type of ribbon 

 is nevertheless invariably found with^assa, Gydona^sa^Pusio- 

 stoma (Sw.), Myristica, and numerous other genera strictly ap- 

 pertaining to the family. But if Ricinula^ Planaxisj Cassis^ 

 Columbella^ and Oliva are placed in the same category, the 

 dental characters will assuredly be found wanting; for the 

 genera just mentioned cannot, with any justice to zoological 

 science, be distributed into less than five distinct families apart 

 from Buccinidge. 



I conclude therefore that, until all the families of the Gas- 

 teropoda have been sifted in this way, superfluous families re- 

 jected and nature's own families found, we cannot be in a 

 position to affirm that the dental characters are not in all cases 

 to be depended upon — though this may be in reality quite 

 true. 



If it is intended, by the assertion that the " teeth are essen- 

 tially epithelian cells," to lessen their morphological importance 

 and convey the idea of mutability, surely we ought to be able 

 to draw a distinction between the normal and abnormal deve- 

 lopment of the same organs in different members of the same 

 species. Moreover the teeth are formed from a special matrix 

 at the fundus of the lingual sac, determining in every case the 

 constant evolution of certain characters ; and any defect in the 

 formative pulp will repeat any consequent malformation in 

 each succeeding row of teeth. 



If it is implied that, as it were, obedient to circumstances 

 affecting adaptation, nature may possibly give a buccinoid 

 ribbon to a veritable Murexy such a doctrine is quite untenable. 

 My own impression is that the distinctive characters of the 

 teeth are in accordance with a definite plan, whether we asso- 

 ciate this with the adaptation of the creatures to the special 

 conditions of their existence or not. Any one finding Helicina 

 (a truly terrestrial Nerite) and Helix (an inoperculate bisexual 

 snail, with a broad lingual pavement) feeding in a tropical 

 forest under precisely similar circumstances, would be inclined 

 to yield the palm to the plan rather than to the conditions of 

 existence. On Mr. Darwin's beautiful hypothesis, the diver- 

 gence of species from a primitive type may be readily admitted; 

 yet, like the coloured components of white light diverging 

 from the prism, it would be unphilosophical to suppose them 

 capable of reciprocally interchanging their characters and 

 properties, even if it were possible to refer all to a common 

 source. 



No one would approve of establishing the peculiarities of the 

 dentition of Mammalia or of any other great class of animals 

 as a kind of ready reckoner of affinities, without taking into 



17* 



