100 



Mr. M. C. March on the 



Anterior to tlie ligament : — 



LA I, LA TU, LA V, for the riolit valve. 

 LA II, LA IV, LA VI, for the left valve. 



Posterior to tlie ligament : — 



LP I, LP III, LP V, for the right valve. 

 LP II, LP IV, LP VI, for the left valve. 



The posterior ends of these lamelloe bend round so as to 

 He more or less perpendicular to the hinge-line, and may 

 become differentiated from the anterior part. The posterior 

 portions form ihe cardinals, the anterior jiortions form the 

 lateral teeth or remain as unditfercntiated lamellae. The 

 posterior himellai are unaltered except in the case of Condylo- 

 cardia (fig. 7) . The anterior cardinals may become bifurcated 

 forming anterior portions. 



rig. 7. 



CAZ 



f.P3 



VoKh/hcardia crassicosta (after Bernard), showing the occuiTeuce of 

 posterior cardinals. 



1. Left valve. 2. Pight valve. 



The reduction of the heterodont hinge to a scheme, and 

 the cursory comparison of that scheme with the actual adult 

 heterodont hinge, makes the conception seem too simple to 

 be really possible. It is only by carefully follov\'ing out 

 Bernard^s papers, and by the comparison of his descriptions 

 of the adult shells with the actual specimens that it becomes 

 clear that the hinges do develop on that plan. In actual 

 practice Y appears rarely, ami YI very rarely. 



