88 AQUATIC MICROSCOPy FOR BEGINNERS. 



the observer to suppose that some extraneous and 

 foreign object has been accidentally included in the 

 mount. 



Besides the ordinary markings on the valves — that 

 is, the transverse lines which are sometimes so coarse 

 that they are called ribs — each valve frequently bears 

 a line or, a narrow smooth . band down the middle, 

 named the raph6, while at each end and at the center 

 there is often a small rounded spot resembling a circu- 

 lar space, but being in reality a thickening, and called 

 a nodule. 



Immense beds of fossil frustules are found in many 

 parts of the world, especially in b'ur own country. In 

 Maryland and in New Jersey diatomaceous earth is 

 obtained containing exquisite forms. In Virginia a 

 certain deposit is especially renowned, since it is 

 eighteen feet thick and underlies the city of Rich- 

 mond. This has afforded the student some of the 

 rarest and most valued frustules, or valves, for the 

 frustuje, before its sculpturing can be properly 

 studied, must be separated into its two valves. To 

 have produced such a rnass they must have existed in 

 incalculable numbers in a great body of watex where, 

 undisturbed for a long time, they died and sank to the 

 bottom year after year, their skeletons accumulating 

 as others continued to fall. To appreciate the prob 

 able length of time during which they existed 

 with nothing to interrupt their peaceful life, 

 as well as the number of diatoms needed to make 

 such a deposit, it is only necessary to know that a sin- 

 gle frustule is seldom thicker than the one ten-thous- 

 andth of an inch. 



At the present day living diatoms are aften found 



