44 FORAMINIFERA. 
This genus of fossils very nearly resembles Delphinula. The 
main difference appears to be that the whorls do not increase so 
rapidly in size in the former as in the latter. Fig. 350. 
EXOGY’RA. Sow. A genus of fossil bivalves, resembling Chama 
in shape and Ostrzea in structure. Fig. 183. 
EXTERNAL. An external shell is one which contains the 
animal, and is not covered by the mantle. 
FASCIOLA’RIA. Lam. Fam. Canalifera, Lam. Siphonosto- 
mata, Bl. Deser. Elongated, fusiform, ventricose ; spire conical, 
consisting of few, rounded or angulated whorls; aperture wide, 
terminating in a long, straight, open canal; columellar lip with 
several oblique folds, the lower of which is larger than the rest; 
operculum horny, pyriform. Obs. This genus is known from 
Fusus by the folds on the columella; from Turbinellus, by their 
obliquity. Fig. 386, F. Trapezium. 
FAU’NUS. Montf. Metanopsis, Fer. 
FERUSSINA. Grateloup. Srropxostroma. Deshayes. 
FI’BROUS. A shell is said to be of a fibrous structure when a 
fracture would present a series of perpendicular fibres, as Pinna. 
FI'CULA. Sw. A generic group of shells, consisting of those 
species of Pyrunta, Auct. which have the true pear-shaped 
character. Fig. 390, P. Ficus. Sowerby confines the name 
Pyruta to these species. 
FIM’BRIA. Megerle. Corzis Fimbriata, Lam. 
FISSUREL’LA. Brug. (Fissura, a fissure.) Fam. Calyptracea, 
Lam. Branchifera, Bl. Descr. Patelliform, oval or oblong, 
radiated; apex anterior, perforated. Obs. The Fissurellz are 
known from Patellee by the perforation in the aperture. 
FISTULA’'NA. (Fistula, a pipe.) Fam. Tubicole, 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 Gastrochena by the straightness of the tube, and 
the oblong shape cf the valves. Fig. 53, Fistulana Clava. 
FLORIL'LUS. Montf. A genus of microscopic Foraminifera. 
FO'LIATED. (Foliwm, aleaf.) When the edges of the successive 
layers of which a shell is composed, are not compact, but are 
placed apart from each other, projecting like tiles on the roof of 
a house, the shell is said to be of a foliated structure. The 
common Oyster, fig. 180, presents a familiar example. 
FORAMINIFERA. D’Orb. An order established for the 
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 Dujardin, on comparing the fossils with some recent 
species of thesame class, arrived at the conclusion, now generally 
adopted, that they constitute a distinct class, much lower in 
