EVOLUTION 109 



Ephippodonta, which is commensal in the burrow of a 

 species of Prawn (Axinus) ; Scioberetia, which is a 

 parasite in the ambulacral zones of an incubating 

 Echinoderm (Tripyhis) ; and Entovaha, which is 

 parasitic within Synapta. 



No instance of a shell-less Bivalve has as yet been 

 recorded. While, therefore, it is not so pronounced 

 as in the case of the Gastropoda, there is still 

 evidence of an increasing tendency in the Pelecypoda 

 towards the reduction of the shell as one proceeds 

 from the more primitive to the more specialized 

 forms. 



One feature in connection with the Bivalve Shell 

 there is that distinctly shows a tendency to simplifica- 

 tion, and that is the progressive reduction of the 

 number of teeth in the hinge. The oldest forms, 

 such as many of the Palseoconcha of Neumayr, the 

 more archaic living forms (Nuculidse, Arcidae, etc.), 

 and the embryo shells of many higher forms (Os- 

 treidse, Pteriidae, Philobryidae, Mytilidas, etc.) exhibit 

 a more or less rectilinear hinge-line with numerous 

 small teeth (Taxodont). In the yet more advanced 

 forms (Condylocardia and Scioberetia) this stage, 

 present in the early embryo, is succeeded by the 

 series of folds (characteristic of the young stages of 

 the higher Pelecypods) that subsequently divide off 

 into cardinal and lateral teeth, thus linking the 

 Taxodont with the Heterodont and Desmodont types 

 of hinge. In these last groups the hinge teeth pro- 



