MARINE DEPOSITS. 



151 



. Cm'tAiwr 

 muta'bile. 



Fig. <244.Mu'rex 

 alveola' tus. 



Fig. 245. Volu'ta 

 at/lie' ta. 



Marine bivalve shells generally differ very much from those 

 found in fresh water ; some resemble oysters, and others are almost 

 entirely like them ; a great many are furnished with ribs, or striae, 

 or rugosities (figs. 24ti, 247), and possess, in a word, many cha- 

 racteristics entirely different from those found in the genera we 

 have just mentioned. 



Fig '.'46. Chama 

 folia' c fa. 



Fig. 247. Ve nerica rdia Fig. 248. JVaw' tilus 

 imbrica'ta. trunca'tus (from the Lias] 



Chambered shells are found only in seas, such as the nautilus 

 (fig. 248), more or less like numerous species of ammonites (Jig. 

 249), no analogue of which is now living, but with which certain 

 terrestrial strata are filled. 



These deposits are generally formed very slowly, by the accumulation of 

 ^llusks as fast as they perish, and not by sudden catas- 



lnvf> Knor\Af1 tl-icim . K.* A i . ._ _T _ - L __ 



I oy dead rno**u.c.o u,o IO.OL ao tucjr pcnsii, ana not oy suoocn etnas 

 U-ophes, which would have heaped them up alive in greater or less numbers 



34 



