FISSURELLA. Brug. (_Fissura_, a fissure.) _Fam._ Calyptracia, Lam. Branchifera, Bl.--_Descr._ Patelliform, oval or oblong, radiated; apex anterior, perforated.--_Obs._ The Fissurellæ are known from Patellæ by the perforation in the apex. Fig. 245. The catalogue published by the author in the Conchological Illustrations, enumerates 68 species. FISTULANA. Lam. (_Fistula_, a pipe.) _Fam._ Tubicolæ, Lam. Adesmacea, Bl.--_Descr._ A transversely elongated, equivalve, inequilateral bivalve, enclosed by a septum within the widest, closed extremity of a straight calcareous tube. Fistulana is known from Gastrochæna by the straightness of the tubes, and the oblong state of the valves. Fig. 54, Fistulana Clava. FLEXUOUS. Having windings or bendings. _Ex._ The Tellinæ are known by the twist or flexuosity in the posterior ventral margin of the shell. FLORILLUS. Montf. A genus of microscopic Foraminifera. FLUVIATILE. (Fluviatilis.) Belonging to a river or running stream. _Ex._ Limnæa 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. 180, 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 interesting to form a distinct branch of study, I do not think it desirable to describe the genera, or to present any arrangement of them in this work. FORNICATED. Arched or vaulted, as the exfoliations on the costæ of Tridacna Elongata, fig. 157.