rEONDTCULABIA. 157 



a fibrous structure, composed of rhomboidal particles. An 

 example of this will be found in the Belemnites, which if broken? 

 shew the perpendicular fibres. In other cases, the matter which 

 has entered and filled up the cavities of the shell has become 

 silicified, or changed into flint, and the shell itself has been 

 decomposed and fallen ofi", so as to leave nothing but an external 

 or internal cast of its form, in flint. This is called a Conchy- 

 liomorphite by continental writers. Some of the most important 

 of Geological data are obtained by a minute comparison of fossil 

 shells, found in various beds, with recent ones presenting the 

 nearest resemblance to them. Some species of fossil shells are 

 considered as identical with recent species. And many G-eo- 

 logists seek to fix: the chronology of the different strata by the 

 number of species which they inclose bearing a resemblance to 

 the recent species. Indeed, all who would study Geology with 

 success, will find it indispensably necessary to obtaia a thorough 

 knowledge of Conchology. 



FEAGELLA. Swainson, (1840.) Clanctjltjs, Montf. 1810. 

 A subgenus of Monodonta, consisting of M. Pharaonis (fig. 361), 

 and similar species. 



EEAGILE. (Fragilis.) Tender, easily broken. 



EEEE SHELL. One that is not attached. 



EEEE VALVE. In attached bivalve shells, one only is fixed ; 

 the other is then free, as far as to the action of opening and 

 shutting. 



EEESH-WATEE SHELLS, (sometimes described as aquatic) 

 are those which inhabit rivers, running pools and ditches, in 

 which case they are fluviatile ; or wells and ponds of standing 

 water, &c. Eresh-water shells are either thin and horny in their 

 texture, as the Limneana of Lamarck ; or are covered with a 

 compact, smooth, homy epidermis. They are generally simple 

 in form, subject to corrosion where the epidermis is wounded or 

 broken, and are circumscribed with regard to the classes and 

 genera to which they belong. The family of Nayades includes 

 nearly all the fresh-water bivalves ; and the Melaniana and 

 Limneana are the principal among univalves, 



EEONDICULAEIA. Defr. A genus of microscopic Eoraminifera. 



