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 (Tvipylus); and Entovalva, 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 Palzoconcha of Neumayr, the 
more archaic living forms (Nuculidz, Arcide, etc.), 
and the embryo shells of many higher forms (Os- 
treidz, Pteriidz, Philobryide, Mytilide, 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- 
