284 Mr. W. Clark on Ancylus oblongus and A. fluviatilis. 



dited bases of generic construction, for it is apparent throughout 

 his descriptive notes that almost every species of each genus 

 not only varies in its accessorial garniture, but also often in its 

 fundamental structure. The only instance in which he speaks 

 with anything like impression on the gasteropodan tongues, is 

 on the transference of the minute Murices to his Conidce. Yet, 

 strange to say, though he admits that they, his Man^elia, have 

 the vital organs of the Murices, he removes them from their 

 legitimate station as species of that genus to one different, both 

 as regards the structure of the shell and animal, — to the Con- 

 volutida, on the ground of their dentition approaching that of 

 the Cones. But even here there is error, as some of the 

 tongues of his Mangelics are of muricidal type. Notwithstanding 

 our deference, we think that this great zoologist has here failed 

 to define the true limits of generic composition. 



To attempt a classification on such elements will assuredly 

 induce deplorable results, by the disseverance of groups and 

 families that are now indissolubly united by nature through the 

 identity of every essential organ. The dispersion consequent 

 on such a proceeding cannot fail to dislocate, more or less, every 

 genus, and consign its species to a false malacological position. 

 We need go no further than our present memoir to prove the 

 fallacy of the lingual riband test, which has split our Ancyli into 

 two genera, Ancylus and Velletittj the tongues of which differ 

 materially in composition, whilst every other organ pronounces 

 them congeneric and of true Limneadan extraction. 



We ask, then, can zoologists for a moment put into compe- 

 tition the essential consentaneity of all the important organs of 

 the Ancyli, excepting the details of the lingual riband, and con- 

 tend that the variations in its fabric ought to swamp an identi- 

 cal concatenation of every other source of vitality, and entitle it 

 to become the dictator of natural position ? The supporters of 

 such a doctrine might with more reason have adopted the varia- 

 tions of the nervous ganglia, or of any other organ, for the basis 

 of a new scheme of generic arrangement, instead of the very 

 worst and most fallacious : and even if there were any value in 

 their view, it cannot be used, from the difficulty, in a thousand 

 instances, of being put into practice : then why make the attempt, 

 when we have other means of easier and more sure application, — 

 the comparison of the united organs and physiological states of 

 the different animals ? We think that the tongue of the Gas- 

 teropoda will never exercise a greater influence than as a spe- 

 cialty of excessive variableness. 



The habitat of this species greatly differs from A, oblongus in 

 being rarely attached to aquatic plants, and then only in deep, 

 slow rivers, whilst its congener is never fixed to stones. At 



