9:5 



Mr. :\r. C. March on the 



traced from a period when these internal ribs alternate 

 with the external ribs, and a time when they are entirely- 

 independent of them. The cardinal plateau is a subsequent 

 development to the first-formed teeth and arises on them. 



Fio'. 2. 



Development of Pectunculus obovatiis. (After Bernard.) 



1 & 2, right valve ; 3-7, left valve. Li = primary ligamental pit ; 

 C = baud of crenulations. 



Where these first-formed teeth remain as internal ribs they 

 are called " dysodont/^ when they are developed on the 

 plateau, or take their place on it, they become true taxodont 

 teeth, and as such become capable of growth into the usual 



I 



