TONGUES OF MOLLUSCA. 75 



in a row ; but at present my object is to define the cha- 

 racters they have in common^ and to show that they are 

 perfectly distinct from those in the orders previously men- 

 tioned. The general character of this form of ribbon, 

 then, is that it consists of three longitudinal bands of 

 membrane laid side by side — a central and two lateral 

 ones, the central band being armed with one tooth, and 

 each lateral with either one or three, in the transverse 

 series. In a few families, I may mention, some or even 

 all these teeth are wanting. The specimens by which I 

 shall illustrate this order are tongues of Fusus antiquus, 

 Buccinum undaturrij Nassa reticulata, Purpura patula, Ce- 

 rastoma Nuttalli, Natica monilifera, Littorina littorea, Cy- 

 praa albuginoides, and Luponia vitellus. 



Some of those who have given their attention to the 

 lingual dentition of the Mollusca have expressed doubt 

 as to its trustworthiness as a guide to the essential organi- 

 zation of the animals. Woodward, for instance, says, 

 " 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, most subject to 

 these adaptive modifications, and can never form the basis 

 of a philosophical system." Other authorities hold the 

 opposite opinion quite as strongly; but, after all, the 

 question is not one to be decided by an opinion, but by 

 facts ; for what the conchologist wants to know is whether, 

 as the result of observation, the lingual ribbons can be 

 relied on or not, and if not absolutely, still to what ex- 

 tent. This being the state of the case, it is necessary to 

 move step by step in the inquiry ; and the first point to 

 decide is, whether there is a characteristic form of ribbon 



