412 FOSSIL OSTREID^ OF NOKTH AMEEICA. 



PLATE LXXIII. 



Fig. 1 represents an American oyster in a moderately "fat" condition. The heart 

 of this specimen, after being opened for over twenty-four hours and exposed to the 

 a[r, would still beat feebly if irritated. 



a and a', great aort» or arteries given off at these points from the ventricle ; au, 

 right auricle ; ir, branchio-cardiao vessels; /y, organ of Bojanus in outline; 6jj, bran- 

 chial pores; c, pericardiac memb'.ano of right side thrown back ; cl, clpaca or cloacal 

 space ; d, nervous commissure of the right side connecting the parieto-splancbnic gang- 

 lion pg and the supraoBBophageal ganglion sg ; f, ventral jirocess of the body-mass; 

 g, gills; gc, gill cavity between the mantle leaves; It, grooved hinge-end of the left 

 valve; I, ligament; M, adductor muscle; mt, mantle; mi', portion of mantle adhe- 

 rent to body-mass ; n to z marks the extent to -which the right and left leaves of the 

 mantle are confluent ; p, palps or lips ; p', outer end of pedal muscle of right side ; s, 

 external opening of the generative and renal organs of the right side ; r, vent or anus ; 

 re, ventricle; a;, x,x, cavities in the edge of the shell filled with mud ; y, jiosterior exi- 

 tromity of the gills and junction of tUe leaves of the mantle. 



