319 



Mulleria lohata, a member of the Unionidse, "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 dee]> attached and flatfish 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 Mulleria 



. . . . 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 peetiniform, 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 peetiniform. "It has the well- 

 developed ears, deep bj^ssal 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- 

 ence." 



In genera such as Ostrea and Plicatula, where fixation takes place 

 at the close of the prodissoconch stage, the succeeding stages give very 

 little indication of the ancestry, owing to the extensive modification of the 

 shell as soon as fixation takes place. According to Dall Ostrea is derived 

 from tlie Pteriidae. 



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 nepionic period. Therefore this genus may be expected to 

 afford some evidence of recapitulation. The first nepionic stage of Spon- 

 dylus is decidedly peetiniform. 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 Rhomftopteria, a member of the true Aviculidte, later 

 succeeded by a growth .... bearing marked features referable in 

 origin to an ancestral genus Pterinopecten Still later a stage 



