MARINE DEPOSITS. 



149 



univalve, unilocuJar shells. The bivalve shells of fresh-water de- 

 posits, more rare than the preceding, are like mussels u'nio (Jig. 

 231), anodo'nta (Jig- 232), cy'clas (fig. 233), and cyre'na (fig. 

 234). The entire absence of every species of polypa'ria (figs. 



Kg. 231 - U'nio Fig. 232. 

 littora' Us. cordieri. 



Fig. 233.-Cy'cZa* 

 obo'vata. 



trigo'nula. 



235, 236, 237239), and echini'dese (figs. 238 240, 241), is an 



important characteristic of fresh-water deposits, which are very 

 common in different parts of the world. 



5. Marine deposits. These are distinguished by the analogy 

 o/ the organic remains they contain (figs. 235 to 250) to the debris 



Fig. M5Encri'nites 

 monilifo' rmis. 



Fig. 236. Jl'piocri' nitee Fig. 237. - Cy' aihocri 1 nitet 

 rotu'ndus. planus. 



4. How are fresh-water deposits recognised ? Which are most numerous" 

 ariivalve or bivalve shells, in fresh-water deposits ? What does the absence 

 of polypa'ria indicate? 



5. How are marine deposits distinguished ? What fossils are characte 

 stic of marine deposits ? 



' 13* 



