6o PALEONTOLOGY 



symbolized by the arable numeral i ; the other two are 

 derived from lamina in., and are called 30 (the anterior) 

 and 36 (the posterior). Similarly in the left valve 

 the anterior and middle cardinals are both derived from 

 lamina n.,so they are called 20 (anterior) and 2b (middle), 

 while the posterior comes from lamina iv., and so is 

 called 46. These facts are expressed diagrammatically 

 in Fig. 1 6. It will be noticed that the alternation of 

 the cardinal teeth follows that of the anterior-laterals 

 from which they are derived those numbered 2 fit in 

 between those numbered i and 3 ; those numbered 3 

 between those numbered 2 and 4. 



Heterodont hinges like the one described are known 

 as the Cyrenoid type ; there is another type with 

 fewer teeth, the Lucinoid type, shown diagrammatically 

 in Fig. 1 6. In this, the central tooth of the hinge is in 

 the left valve it is 2, instead of i. The complete dental 

 formula for each may be written thus : 



Lucinoid type ... ,- 



Cyrenoid type . 



A shorter formula, omitting the laterals, gives -W 



2, 40 



for the Lucinoid and ' ^ a ' , for the Cyrenoid type. 

 20, b, 46 



A dentition rarely contains the full set of teeth, but it 

 can still be classed as Lucinoid or Cyrenoid ; the laterals 

 in particular are often reduced in numbers, and when 

 the umbo is far forward the anterior-laterals may be 



