9o ^Ir. ^I. C. IMavcli 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 platean is a subseqneut 

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



Development of Pectitncuhis obovatus. (After Bernard.) 



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

 C = l)and of crenulations. 



A^'here these first-formed teeth remain as internal ribs they 

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

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

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



