EXPLANATION OF TERMS. 405 



Tricuspidafe. When these divisions end in points. 



Truncated. Ending abruptly, as if cut off, as in the 

 extremities of the typical Soleus, Mya truncata, &c. 

 The same term may he applied to the termination of 

 the spire in several land-shells, which seem as if 

 they had been broken off. 



Tuhivalves. Such bivalves, like Teredo, as are pro- 

 vided also with a tube. 



Turbinated. Turban or top-shaped ; the whorls con 

 vex, and generally but very little raised. The garden 

 snails are good examples_, but not the Trochi, thel 

 form being pyramidical. 



Umbilicus. A hollow opening, more or less wide, 

 on the side of the inner lip of spiral shells : in some, 

 as Scalaria and Sollarium, it is so deep as to ex- 

 tend to the apex of the shell ; in others, as Tro- 

 chus, it is small ; in Bulimus it becomes nearly 

 obsolete ; and in Natica it is often entirely con- 

 cealed by the spreading of the inner lip. In all 

 these variations, however, the umbilicus is caused by 

 the inner edges of the whorls not touching each other. 

 Instances occur in certain land-shells which are 

 deeply umbilicated when young, but not at all when 

 old. 



Undulated. Waved ; nearly synonymous with sinuated, 

 but more particularly applied to the colouring of 

 shells. Scapella undulata and many other volutes 

 are marked in this way. 



Valves. The two ordinary or principal pieces which 

 comprise the covering of acephalous Testacea, or 

 bivalves. 



Varix. Those strong elevated ridges which cross 

 the whorls of several of the predaceous genera, 

 as Murex, Triton, &c, : they mark the progressive 

 enlargement of the shell, being the remnants of 

 former apertures, beyond which the animal, when 

 D i> 2 



