58 PALEONTOLOGY 



teeth, radiating from the umbo the cardinal teeth ; in 

 the anterior and posterior regions of the hinge-plates there 

 are long teeth, more or less horizontal, parallel to the 

 shell-margin these are called (inaccurately) the lateral 

 teeth. The anterior-lateral teeth come close up to the 

 cardinals, but the posterior-laterals are separated from 

 them by a space. The posterior-laterals are always 

 posterior to the ligament this fact is the essential dis- 

 tinction between them and the cardinals, the posterior of 

 which may in many heterodont shells be long and nearly 

 horizontal, so that it might be mistaken for a posterior- 

 lateral if its position relative to the ligament is not noticed. 

 To come to details in the right valve there are two 

 laterals, the inner of which is on the inner margin of the 

 hinge-plate, while the outer one is not quite on its outer 

 margin ; there are three cardinals, of which the middle 

 one is bifid ; and two posterior-laterals, occupying similar 

 positions on the hinge-plate to the two anterior-laterals. 

 In the left valve, there is a well-marked anterior-lateral 

 tooth, not quite on the inner margin of the hinge-plate 

 (since it has to interlock between the two anterior-laterals 

 of the right valve) ; the outer margin of the hinge-plate 

 is very slightly raised into a vestigial tooth. There are 

 three cardinal teeth, each fitting in behind the correspond- 

 ing tooth of the right valve. The posterior-laterals are 

 similar to the anterior-laterals. 



The general arrangement of the teeth is always the 

 same in all Heterodonts : the lateral teeth of the left 

 valve are always on the outer side of (i.e., above) 

 the corresponding teeth of the right valve. This 



