100 



Mr. M. C. Maicli on the 



Anterior to tlie ligament : — 



LA I, LA HI, LA V, for the ri^ht valve. 

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



Posterior to the 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 lamellae bend ronnd so as to 

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

 become differentiated from the anterior part. The posterior 

 portions form the cardinals, the anterior portions form the 

 lateral teeth or remain as undifferentiated lamellae. The 

 posterior lamella are unaltered except in the cuHe of Condy/o- 

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

 forming anterior portions. 



Fig. 7. 



CAE 



LP 3 



('u/uJi/li ((irdid crasskosta (after Bernard), showing the occun-enre of 

 posterior cardinals. 



1. Left valve. 2. Rioht valve. 



The reduction of the hetei'odont hinge to a scheme, and 

 the cursory comparison of that scheme with the actual adult 

 lieterodont hinge, makes tlie conception seem too simple to 

 be really possible. It is only by carefully following 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. lu actual 

 practice Y appears rarely, and YI very rarely. 



