BLOOMER: THE ORIGIN AND FUNCTION OF THE FOURTH APERTURE. 43 



E. ensis 



E. siliqua. 



S. vagina. 



C. pellucidus 



Primitive form. 



Since the puhlicution of Dr. Ridewood's valuable work on the structure 

 of the gills of the Lamellibranchia no review of their classification would 

 be complete without a reference to it ; but as I propose referring to the 

 work in more detail at a later date when dealing with the classification of 

 other genera of the Solenidae, I will only now say that I do not see anything 

 in it disagreeing with the above conclusions. 



ON THE ORIGIN AND FUNCTION OF THE FOURTH 

 APERTURE IN SOME PELECYPODA. 



By H. H. bloomer. 



It has been suggested that the origin of the fourth aperture corresponds 

 with a rudimentary fissure for the byssus. While this may explain the 

 cause of the orifice in certain of the Pelecypoda, I have from observations 

 come to the conclusion that in others the origin is totally dissimilar ; and, 

 further, in those to Avhich this theory may apply it has in certain cases 

 since been utilised for an altogether different function. It is noticeable 



