General Classification of the Pelecypoda. 113 



Order 11. HETERODONTA. 



Pelecypoda in which the prodissoconch normally shon^s 

 no embryonic crenulations. The teeth develop from lateral 

 lamelhe. The succession is external (dorsal). The first- 

 formed lamella is in the right valve. Each valve contains 

 lamelloe in front of and behind the ligament. The anterior 

 lamelhe may develop posteriorly into cardinal teeth and 

 anteriorly into lateral teeth. Posterior cardinals are deve- 

 loped in one case oidy. 



Division A. Pliodonta. 



Heterodonta in which CA 1 is developed and CA 2 is 

 subdivided into anterior and posterior portions. 



Division P. O l i g o d o n r a. 



Heterodonta in which CA 1 does not develop and CA 2 

 remains undivided. 



Conclusion. 



The comparison and contrasts between these three classifi- 

 cations stand out clearly. To Nenmeyer, the first to really 

 tackle the problem of the hinge as a basis for classification, 

 is due the honour of having divided the hinges into types. 

 Three only of these divisions survive, two of these only as of 

 subordinal value (Taxodonta and Dysodonta), the third 

 (Heterodonta) as an order; but to him is due the general 

 basis for such a classification. Dall kept these tj^pes of 

 Neumeyer's, but reduced them to the rank of divisions in 

 his orders. He created three new orders, founding them, as 

 did Nenmeyer, on the characters of the adult shell, and, as 

 was shown by Bernard^s later work, erroneously. Bernard^s 

 work was essentially that of an eml)ryologist. His two 

 orders and their suborders were founded on the study of 

 individual development. Having worked out the main 

 lines of his classification in this way, he compared it with 

 Neumeyer's work on the Palaeoconchs of the Palaeozoic 

 period, and found that the results of his work were borne 

 out by these earlier researches. 



Neither of these workers claimed that his work was ideal 

 phylogenetically ; each fully realized the importance of the 

 consideration of other organs in tracing out the relationships 

 of members of the group, 



Ann. (ii Mag. N. Hist. Ser. 8. Vvl. x. 8 



