72 PAL/EONTOLOGY 



different areas.) The ornament is shown by the internal 

 cast as well as by the outer shell. There is a short, 

 opisthodetic external ligament. In the fossil it is difficult 

 to determine whether teeth are present or not ; but in the 

 recent species there is only a vestigial tooth. The shell 

 is isomyarian and sinupalliate, though the impressions 

 are too feeble for this to be determined in many cases. 



The ten genera of Lamellibranchia here described 

 . form a very inadequate sample of a very extensive class, 

 for which various methods of classification have been 

 from time to time proposed. Classifications are of two 

 kinds morphological and phyletic. All early classifica- 

 tions necessarily belong to the former type, in which one 

 proceeds to sort out the members of a class first into 

 larger groups defined as precisely as possible by certain 

 differences of structure, then into smaller groups defined 

 by other differences, and so on. The important question 

 to be settled in framing a morphological classification is 

 What characters shall we choose to define our larger 

 groups, and what to define each successive grade in our 

 subdivision ? According as the characters are well or 

 ill-chosen, the classification comes to be recognized as 

 more or less " natural " or " artificial." The characters 

 to be considered fall into several categories : 



(i) The most external or superficial characters, such 

 as the shape and ornament of a shell. These are found 

 pften to differ greatly in a number of species which have 

 most other characters alike, hence they are recognized as 

 principally of value as specific distinctions; and only 

 when they are constantly associated with other characters. 



