319 
Mulleria lobata, a member of the Unionide, “is so remarkably like an 
oyster (in the adult) that it has been called the fresh-water oyster. In 
the monomyarian adult . . . . the shell is rough and irregular with 
a deer attached and flattish free valve, and a specimen in the Museum of 
Comparative Zoology is indistinguishable in shape from forms commonly 
found in Ostrea virginiana... . . . The young shell of Mudlleria 
. . . . is Anodon-shaped, equivalvular and dimyarian as described by 
authors.” 
Hinnites is another genus which has the ostreaform adult. “In the 
young it is free and pectiniform, but in the adult . . . . so close 
is the likeness to an oyster that in the synonomy of the genus it has been 
named Ostrea and Ostracites.” In Hinnites cortesi of the Tertiary, in 
the neanic stage, the right valve is purely pectiniform. “It has the well- 
deyeloped ears, deep byssal sinus, and an evenly plicated shell which at 
this stage is nearly or quite equivalvular.” With the period of attach- 
ment a most marked change in the valves takes place and the adult be- 
comes deeply concave (in the right attached valve) and highly ostrea- 
form. The byssal notch is filled up and “completely wiped out of exist- 
e1ice.”’ 
In genera such as Ostrea and Plicatuia, where fixation takes place 
at the close of the prodissoconch stage, the succeeding stages give very 
little indicaticn of the ancestry, owing to the extensive modification of the 
shell as soon as fixation takes place. According to Dall Ostrea is derived 
from the Pteriide. 
Spondylus is another genus in which cementation has caused exten- 
sive modification of the valves in the adult. Fixation takes place at the 
close of the pepionic period. ‘Therefore this genus may be expected to 
afford some evidence of recapitulation. The first nepionic stage of Spon- 
dylus is decidedly pectiniform. It has a long hinge-line and a deep byssal 
sinus. After fixation, in the first stages of irregular growth, the byssal 
notch is soldered over, and eradicated in a manner similar to Hinnites. 
Another illustration of recapitulation among the Pelecypoda is the 
case of Pecten itself. Of this genus Jackson says: ‘In the development 
of the modern Pecten we find in the first stages of dissoconch growth a 
form of shell . . . . presenting characters which make it referable 
in ancestral origin to Rhombopteria, a member of the true Aviculide, later 
succeeded by a growth . . . . bearing marked features referable in 
origin to an ancestral genus Pterinopecten. . . . . Still later a stage 
