MOLLUSCA OR UNIVALVES. 265 



This shell, which is very curious, and not 

 uncommon, is found in the Eastern Ocean. It 

 is thin and fragile, and of a pearly white. The 

 only species known is S. Peronii. 



129. Nummulites. — Shell thin towards the 

 margin; spire internal, discoid; 

 whorls contiguous, and not ap- 

 parent ; cells numerous and 

 small ; partitions transverse, and not perforated. 

 — 4 species. 



These shells are all fossil, and exist in im- 

 mense numbers ; they are very flat, and take 

 their name from their resemblance to pieces of 

 money. In many parts of the world they are 

 accumulated in such vast masses as to form en- 

 tire mountains, and many buildings have been 

 constructed of limestone crowded with them. 

 They vary in size from more than an inch in 

 diameter to great minuteness. 



130. Nautilus. — Shell, whorls contiguous, 

 the last enveloping the others; chambers nu- 

 merous, formed by transverse partitions, and 

 perforated by a tube. Animal having numerous 

 tentacula, and attached to the shell by two 



