THE LAMELLIBRANCHIA 73 



can they be taken as part of the definition of larger 

 groups. 



(2) Characters which are evidently directly connected 

 with the animal's particular mode of life adaptative 

 characters. These have to be taken very cautiously, since 

 sometimes two animals from widely different groups may 

 come to resemble one another very closely (especially 

 when we have only the hard parts to deal with) on account 

 of their adaptation to similar conditions (convergence}. We 

 have already seen how many features lamellibranchs 

 and brachiopods have in common owing to general simi- 

 larities of habit, and cases of homceomorphy among 

 brachiopods have been mentioned. Among lamelli- 

 branchs themselves there are cases of similarity of 

 external form (e.g. Trigonia and Crassatella), accom- 

 panied by great differences in internal structure. Never- 

 theless in many cases adaptation to different lives does 

 form the basis of such fundamental divergence as to 

 determine essential boundaries in classification, as for in- 

 stance between the fishes and air-breathing Vertebrates. 



(3) There are certain characters which are found to 

 run, with but slight modifications, through forms of 

 varied life-modes and varied external appearance. Such, 

 for instance, are the various types of hinge-teeth in 

 lamellibranchs. Such characters are found to provide 

 the safest bases for classification. 



The history of taxonomy (i.e., the laws of classification) 

 shows that morphological classification tends more and 

 more away from what is regarded as " artificial " 

 towards what is " natural," and this tendency has meant, 



