IN DESCRIBING SHELLS. 29 



stance Solen, Tellma, Venus, Cardiinn, Mytiliis, and Pinna. 

 In several it is internal, as Mactra, Spondylus, Ostrea, 

 Anomia, and most of the Mi/a; : in a few species it is sunk 

 in a g-roove into the thickness of the shell, so as not to be 

 called properly either external or internal, as in Venus Meroe 

 and tigerina,and some species of Donax. In the last division 

 of the genus Ostrea it is di\ ided into many parts, as Ostrea 

 Isognommn, &c. 



Dentes, the teeth, are eminences or projections on the internal 

 surface or margin of the valves, near to the beaks and car- 

 tilage ; they are most generally received into corresponding 

 hollows in the opposite valve : when this is the case they are 

 said by Linnsus to be inserted ; but when there are no corre- 

 sponding hollows to be perceived in the opposite valve, they 

 are said not to be inserted. 



Cai'dinis Dentes, or hinge teeth, are the teeth which are placed 

 in the centre, under the beaks ; those which are found at 

 some distance, on each side, are the lateral teeth ; but in the 

 genus Area there is no such distinction, the teeth being 

 numerous and arranged in a line. 



Siibulatus is when they are long and sharp, or awl-shaped, as 

 in the genus Solen. 



Acutus is when the teeth are short and sharp, as in the genus 

 Area. 



Recurvus is when the teeth are hooked or bent backward, as 

 in Sjiondylus. 



Duplex is when a tooth is as it were split into two. 



