BRITISH NUDIBRANCHIATE MOLLUSCA. 39 



derived from the tongue in this order, we have come to the conclusion that it can scarcely 

 be considered of more than generic importance ; but, taken in connexion with other characters, 

 especially with the presence or absence of jaws and prehensile collar, it will undoubtedly be 

 found a valuable guide, and will often determine an alliance or a distinction where external 

 indications fail ; but like all other characters taken from a single organ, it is liable to lead to 

 artificial grouping when carried too far. 



The arrangement now adopted in this work is, as will be seen by the following 

 Synopsis, an enlargement of that provisionally given in our first part, and restated in a 

 previous page. Several genera and a large number of species have since been added, and 

 some slight modifications have been made in the different groups. The plan of the work 

 would not allow of any extensive alterations, and happily such were not required ; though, 

 could we have done so conveniently, we should have been inclined, with our present know- 

 ledge, to have raised our sections of Doris and Eolis to the rank of genera, as has been 

 already done by Dr. Gray. The time has scarcely arrived when a complete general arrange- 

 ment of the Nudibranchiate order can be undertaken with any degree of satisfaction. As a 

 contribution to such an object, however, we have ventured to give the outline of a general 

 arrangement on an enlarged basis, in our Appendix, a comparison of which with the systems 

 of other authors here given, will serve to show how far our views coincide with theirs, and 

 prevent the necessity of any extended critical remarks on the labours of our brother 

 naturalists. 



