150 FRESH-WATER SHELLS. 



in which it lies. In some cases, the mineral composition of the shell 

 is so completely changed as no longer to present its proper structure, 

 consistingof successive oblique layers of shelly matter; but is altered 

 into a fibrous structure, composed of rhomboidal particles. An exam- 

 ple 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 off, so as to leave nothing but an external or internal 

 cast of its form, in flint. This is called a Conchyliomorphite 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 resem- 

 blance to them. Some species of fossil shells are considered as 

 identical with recent species. And many Geologists 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 obtain a thorough knowledge of 

 Conchology. 



FRAGELLA. Sw. A sub-genus of Monodonta, corresponding 

 with the genus Clanculus, Montf. consisting of M. Pharaonis 

 (fig. 361), and similar species. Sw. p. 352. 



FRAGILE. (Fragilis.) Tender, easily broken. 



FREE SHELL. One that is not attached. 



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

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

 shutting. 



FRESH-WATER SHELLS, (sometimes described as aquatic) are 

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

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

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

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

 compact, smooth, horny 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 



