CONCHOLOGICAL TEEMS. 243 



insertion of the muscle into the valve. It is single or double, 

 accordingly as the animal possesses one or two muscles ; and 

 varies in shape, being circular, ovate, lunate, elongated, &c. 

 The concliifera, as they respectively possess one or two 

 muscles, are termed monomyaria and dimyaria. 



Stria are fine thread-like lines on the external surface of 

 shells. They are sometimes both longitudinal and transverse 



If both valves are of the same size, the shell is said to be 



Wf t/Wl/t/L/ 



If one valve is larger than the other, it is said to be 



If both sides are equal, it is termed equilateral ; if one is 

 larger than the other, it is called inequilateral ; while the 

 prefix sub, means nearly, thus, subequivalve, signifies nearly 

 equivalve, &c. 



THE TEETH. The teeth constitute an important struc- 

 ture, since they serve for the definition of generic characters. 

 They are cardinal, that is, placed in the centre ; or lateral, 

 that is, diverging from the umbones ; they also vary exten- 

 sively in shape and direction, being incurved, or bent round, 

 recurved, or turned back, large or small, numerous, few, 

 rounded, flattened, &c., &c. 



The function of shell-secretion is performed by the 

 glandular free margin of the mantle. Like the shells of 

 conchifera, those of the gasteropoda are sometimes composed 

 of several pieces, as in the chitons. (Fig. 10.) 



The shells of the great majority of gasteropoda are rolled 

 obliquely in consequence of the unequal development of the 

 two sides of the body of the animal. They then form a helix, 

 or an oblique spiral. Sometimes the coil is towards the right ; 

 at other times it is towards the left side : the direction of the 

 spiral is in general constant in the species. 



Some shells have a patelloid form, and are symmetrical 

 without being spiral ; and there are various intermediate 

 forms by which these two extremes, the patelloid and spiral, 

 are blended into each other. Some shells vary much in form 

 at different periods of growth, as shown in the beautiful 

 cyprcGcdssis rufa found by Mr. Stutchbury among the reefs 

 of Paumotus, in the South .Pacific,* (fig. 166), where 



* Magazine of Natural History, vol. i., new series. 



