72 DR. T. ALCOCK ON THE 



tending to show that a steady prosecution of still further 

 researches is likely to lead to most valuable results. 



This being the case, I have devoted my spare time for 

 more than a year past to the dissection of MoUusca ; and 

 one or two of the results_, as regards lingual dentition, I now 

 propose to lay before you. 



The lingual ribbon is found in all the Gasteropoda, that 

 is, in all those Mollusca which, like the Snail, crawl on a 

 broad, flat disc, forming the lower surface of the body ; and 

 the special characters which are found to belong to the 

 organ in dififerent kinds of Gasteropods have been applied 

 to classification for the three following purposes : — to dis- 

 tinguish between very nearly allied species, to determine 

 the true limits of genera, and to form these genera into 

 natural groups or families. 



The orders into which the class Gasteropoda is divided 

 were well established by Cuvier on the characters of the 

 breathing-organs, and, according to his arrangement, 

 they were eight in number; but these have since been 

 thrown together by Milne-Edwards into three groups, 

 founded upon certain broader features of the same organs, 

 which are still admitted by all conchologists to furnish 

 satisfactory distinctions for the division of the class into 

 orders. 



The series of specimens to which I wish to call your 

 attention this evening belong to two of the three orders 

 of Milne-Edwards, namely, the Prosobranchiata, or those 

 which have the gills in front of the heart, and the Pul- 

 monifera, or those which breathe air ; and the first thing 

 I have to remark about them is, that you will find, on ex- 

 amining the series, that, while they all difier considerably 

 from one another, they form themselves into four very 

 distinct and natural groups, with characters so well marked 

 that there cannot be a moment^s hesitation in deciding 

 to which of the groups any specimen belongs. 



