59. 



of Yoldia and the veliun of other forms. In this connection it 

 is of interest to notice that in a few forms the velim is known 

 to bo cast away. 



niis has been obser^;-ed by Heatla# for Cliiton, Siger- 

 foos (19) for Teredo, and Mrs. Drew for Cardium. 



Wilson (^^) has described a sinilar casting of the 

 cells of the velum for Polygordius. 



It seeras ffaite possible that the embryo of Yoldia repre- 

 sents a rather primitive forrn, and tliat the topical molluscan 

 larvae may have been developed from an ancestor approaching it in 

 form and structure. If this is true, the probability is that 

 Yoldia and its relatives, when compared with other lamellibranchs, 

 would represent a comparatively recent departure from the primi- 

 tive molluscan form and, in s\ich a case, it is more likely that 

 organs iifiiioh now show high specialization, have been developed 

 from the more primitive than from the highly complex. 



# Dr. Heath has kindly allowed me to notice this geint al- 

 though his obser\rations are not yet published. 



1 This work has not been published. The species upon 



■mSiich the observations were made seems to be C. pinnu- 

 latuun (Con.) 



