TERMS- UNIVALVES. 13 



outer lip on the right-hand. Occasionally this 

 order is reversed, but not commonly. 



" The operculum, or lid, in univalves is that 

 part which fits exactly into the aperture, and in- 

 closes the animal ; it serves as a door to the shell. 

 (Plate 3.) The operculum is either horny, like 

 that of the periwincle, or of a harder substance, 

 like the shell itself 



" The umbilicus is a circular hole in the body 

 of the shell. This perforation produces a very cu- 

 rious effect when it is very large. When the um- 

 bilicus is wanting, the shell is called imperforate. 



" If the spire is truncated, it is decollated ; if 

 it is surrounded with spires, it is coronated, or 

 crowned. 



" Second section, without a regular spire. 

 The teeth in univalve shells, as the cowry, are 

 ridges upon the aperture. (Plate 1.) InVoluta 

 they are regular folds or plaits upon the colu- 

 mella. 



" K Jissure is a notch or- slit, as in Patella 

 Jissura. (Plate 3.) 



" Some shells of this section are internally 

 lipped, as Patella equestris ; {Calyptrcea of 

 other authors, Plate 3 some are chambered, 

 as the slipper-limpet ; some ar ecap-shaped, 

 having the apex much curved — these are the 

 cap-limpets. 



