FOSSIL SHELL. 149 



from Gastrochsena by the straightness of the tubes, and the oblong 

 state of the valves. Fig. 54, Fistulana Clava. 



FLEXUOUS. Having windings or bendings. Ex. The Tellinse 

 are known by the twist or flexuosity in the posterior ventral mar- 

 gin of the shell. 



FLORILLUS. Montf. A genus of microscopic Foraminifera. 



FLU VIATILE. (Fluviatilis.) Belonging to a river or running stream. 

 Ex. Lirnnsea fluviatilis. 



FLUVIATILE CONCHACEA. See Conchacea. 



FOLIATED, or FOLIACEOUS. (From folium, a leaf) When the 

 edges of the successive layers of which a shell is composed are 

 not compacted but placed apart from each other, projecting like 

 tiles, the shell is said to be of a foliated structure. The common 

 Oyster, fig. ISO, presents a familiar example. 



FORAMINIFERA. D'Orb. {Foramen, a hole or pit) An order 

 established for minute many chambered internal shells, which 

 have no open chamber beyond the last partition. Lamarck, 

 D'Orbigny, and other writers have placed them among the 

 Cephalopoda in their systems, but Du Jardin, on comparing the 

 fossils with some recent species of the same class, arrived at the 

 conclusion, now generally adopted, that they constitute a distinct 

 class, much lower in degree of organization than even the Radiata. 

 Not recognizing these microscopic bodies as shells, properly so 

 called, but considering them sufficiently numerous and inter- 

 esting to form a distinct branch of study, I do not think it desi- 

 rable to describe the genera, or to present any arrangement of them 

 in this work. 



FORNICATED. Arched or vaulted, as the exfoliations on the 

 costee of Tridacna Elongata, fig. 157. 



FOSSIL SHELL. A shell is considered to be in a fossil state when, 

 the soft parts having ceased to exist, it is deprived of all its animal 

 juices, has lost all, or nearly all its natural colour, andis thus changed 

 in its chemical composition, when little or nothing is left but a mere 

 bone, which is embedded in a sedimentary deposit. In this state, it 

 is fragile, prehensile to the tongue, and either destitute of colour or 

 tinged with the diluted mineral matters which pervade the stratum 



