53. 



found in tlie Nuculidae an.d Solenomyidae. On tne otlier Iiand sev- 

 eral recent autliors have considered eitlier Avicula , or Melea.irrina 

 and Area , as representing the most ancient forms of this group. 

 But, it is bo be noticed that Avicula and Melea.frrina 

 have already lost their anterior addiictor (this in itself is not 

 a sign of great antiquity) and further that these free forms have 

 their gills more specializ.ed than the Nuculidae and Solenomyidae. 

 It is not only by tiie appearance of their respiratory apparatus, 

 but by the existence of all the following common characters, 

 that are not found in otlier Lamellibranchs (include here Avicula. 

 Melea^^rina and Area) that these t^ro farailies are undebateably 

 shovm to be the most archaic of ail : 



1. ^ypobranehial gland situated dorsal ly, between the 

 mantle and the gills. 



2. Foot has a ventral "creeping" disc (not existing 

 in others except Pe^tunculus ) . 



3. Gills tCkx posterior, have free, flat, filaments, 

 without reflected lamellae, analagous to the bipectonate gills of 

 Amphineura, Cephalopoda, Bliipidoglossus, and Euthyneura. 



4. Heart posterior, has auricles 5-n contact with the 

 gills only by their anterior extranities and not along their ^^tlole 



axes . 



