466 TERMINOLOGY OF 



are in a very few genera, as Pholas and Xylophaga, two or 

 more calcareous pieces. These are called accessary valves. 



The hinge, or cardinal margin of the valves is very vari- 

 ously modified, and offers many important characters to the 

 conchologist to guide him in the classification of the genera. 

 It is formed out of the inner layer of the shell. When it is 

 a mere thickened rim, or a pit-like callus, we say the hinge 

 is edentulous, as in Pholas, Pecten, and Ostrea ; but there is 

 usually found on it a number of small projections called den- 

 ticles 9 or teeth, and some excavations called pits, fossce, or 

 furrows the teeth of one valve being, in general, exactly 

 adapted to lock into opposite fossae, or gutters, in the other 

 valve, or into the spaces between the teeth. When situated 

 immediately under the beaks, the teeth are named the cardi- 

 nal, or hinge teeth; and when they are removed from the 

 centre and placed on either the anterior or posterior slopes, 

 they are said to be lateral. (Fig. 86 and 87.) The anterior 



Figs. 86, 87. 



lateral tooth is generally situated at the extremity of the lu- 

 nule ; and the posterior lateral tooth at the extremity of the 

 ligament. The teeth vary in number one, two, or three, 

 and sometimes the number is considerable. In the latter 

 case they are described as being serial, or the hinge is multi- 

 denticulate, as in the Arcadse. The terms used to designate 

 the form and direction of the teeth need no explanation, 

 being used in their ordinary sense. 



In some bivalves the hinge is so firmly soldered that it 

 admits of no opening and closing of the valves, or to only a 

 very slight extent; in these, the hinge is connate, or coalite. 



In the family Osteodesmes of Deshayes, the hinge is fur- 

 nished with a moveable ossicle associated with the teeth. It 

 was first observed by Dr. Turton, in Lyonsia striata, who 

 considered it to be a tooth. " This tooth is not a fixed 

 projection from either of the valves, nor formed from the 

 substance of the shell itself, as in all other known shells fur- 

 nished with teeth ; but is an independent process moveable 

 with the ligament, and may be entirely detached from either 

 or both the valves : consequently when the valves are opened, 

 it is found sometimes in the right valve and sometimes in the 



