92 Mr. M. C. March on the 



leads to the development of new species or genera. Their 

 broad changes must be looked upon as being purely intrinsic 

 and as common to the whole group as such. 



On the other hand_, the primitive Pelecypod ancestor is 

 conceived to have been hiugeless. The evolution of the 

 hinges must therefore have taken place entirely within the 

 group. Its development was due to the necessity for securing 

 rapid and accurate closing of the shell, as was pointed out 

 by Dall. Such an end might be assured in many ways not 

 of necessity related to each other, as the hinge has no 

 ancestral form common to the whole group. So that the 

 development of the hinge apparatus should be of taxonomic 

 value, as similarity of development would show a close 

 relationship between subdivisions, and not merely a common 

 membership in the group. The fact that the variations are 

 extrinsic leads to the occurrence of lieterogeneric homoeo- 

 morphy, but such cases should be distinguishable by the 

 study of ontogeny and phylogeny. 



Modern CAassifications based on the Hinge. 



A. Neumkyer. 



Neumeyer was the first, after Martini, to classify the Pelecypoda on 

 the characters of the hinge ; he recognized six orders, founded on distinct 

 teeth characters. 



1. Cnjptodonta. Including forms without teeth or with folds which 



involved the whole thickness of the valve and which were often 

 continuous with the radial ribs. 



2. Taxodonta. Including forms where, in the simplest cases, the teeth 



were perpendicular to the hinge-line, but which might become 

 more or less oblique peripherally. 



3. Heterodonta. Including the most highly specialized of the Lamelli- 



branchs in which the teeth were distinguishable into cardinals and 

 laterals. 



4. Schizudonta. Including those forms which possess one bifurcated 



tooth in the left valve, fitting into two divergent lamellae in the 

 right valve. 



5. Vesmodonta. Including forms very similar to the Heterodonta in 



anatomy, but with an internal ligament, and teeth not homo- 

 logous. 



6. Dysodonta. Containing those Heteromyarians and Monomyarians 



that have partially or totally reduced teeth. 



B. Fischer. 



Fischer added a seventh order to Neumeyer's six : — 



Isodonta, included by Neumeyer in the Desmodonta, contains those 

 forms which have their teeth symmetrically arranged about an 

 internal liuaraent. 



