135 



close in straight double rows ; the mouth round ; the vent 

 oval, the shell round it being vaultedly convex. — Leske ap. 

 Klein, Tab. xiii. fig. A. B. Fossil. 



2. G. depressus. — Circumference circular ; the division 

 of the areas marked by the sutures, but the assulae not 

 shown ; the mouth rather small ; the vent large and oval. 

 Fossil. 



3. G. vulgaris. — Under this vague and comprehensive 

 designation are placed those numerous small fossil specimens 

 which are common in many parts of the world ; possessing 

 the characters of the genus, but varying so indeterminately 

 in their minor distinctions, as not to allow of their being 

 described as even definable varieties. They are found, 

 sometimes, to vary so in their figures, as if they were the 

 members of some undescribed genus : instead of possessing 

 the complete conical form, the vertex is depressed as if 

 truncated ; and sometimes the slight angles are entirely 

 removed, and the sides are so rounded that the fossil pos- 

 sesses almost a globular form. Even the mouth and vent, 

 though in their regular situation, are frequently found to 

 vary much in their size and form. The areas vary con- 

 siderably, being sometimes raised, other times sunk ; some 

 bear the marks of the assulae, and others not. The ambu- 

 lacra vary in their breadth, as well as in their depth or ele- 

 vation, and sometimes, but rarely, show the two lines of pores. 



As these fossils, which are almost all siliceous casts, 

 bear the markings of the inner surface of the shell which 

 they filled, their surfaces must necessarily differ from the 

 outer surfaces of the shells from which they proceeded ; and 

 endless, indeed, are the differences which have resulted 

 from the various degrees of correctness in the casts, and 

 which must have depended not only on the condition of the 

 mould, but on the state of the injected matter and the 

 degree of perfection to which the crystallizing process had 

 been allowed to proceed. 



