EXPLANATION OF TERMS. 25 



Secondly, of the Form. 



Bivalve shells are said to be — 

 jEquivalvis, when both the valves or pieces are equally con- 

 vex, as in Cardkim edule. 

 Inceqidvalvis, when one of the valves is flatter than the other, 



as in Ostrea edulis. 

 Convexus, when tlie two valves are considerably convex or 



swelled out : this is opposed to comjn^esstis. 

 Gihhus, when the valves are considerably but not regularly 



swelled out, being as it were bunched out more in one place 



than in another. 

 Coinpressus, when the valves are but little convex or swelled 



out. Some few are quite flat, as Anomia Placenta ; but most 



shells are more or less convex. 



OF CONVEX FORMS. 



Globosus is when the two valves together form nearly a sphere 



or globe, as Tellina cornea. 

 Ovatus is when the two valves form nearly an egg-shape, as 



Cardium serratum. 

 Ovalis is nearly egg-shaped, but having the two ends equal or 



almost equal, as Pholas cHspata. 

 Oblongus is when the shell is drawn out longer in proportion 



to the thickness, as Pholas candidus. 

 Cylindricns is when the valves together nearly form a cylinder, 



as Mytilus lithophagus. 

 Lmearis-7'ectus differs little from cylindrical ; it is long and 



