1903.] NATURAL SCIENCES OF PHILADELPHIA. 3 



protoplasmic currents causing the motion of diatoms. In Navicula 

 rhomboides it becomes apparent that the raphe is single as in other 

 species of this genus, and that the double raphe, based on which this 

 form has been placed in a separate genus, is an optical illusion. 



This process differs from that by which the late A. Y. Moore plated 

 diatoms, as his were covered with a hea\'y layer of silver or gold and 

 examined as opaque objects. It is rather a staining process, but like- 

 wise differs from the methods of Moore and others in this direction, 

 which filled the cavities in the valves with opaque matter, while in this 

 case the silica itself is rendered opaque, or nearly so. 



As in all microscopical investigation, it is necessary to use judgment 

 in interpreting appearances seen, as they will vary materially in the 

 same species, depending on whether the entire structure is preserved. 

 Isthmia affords an excellent example. Some valves may be found in 

 which the coarse markings appear as holes in the blackened sihca, 

 showing that the secondary structure is entirely gone. Others wall 

 show the secondary structure in a similar manner as holes in a darkened 

 plate, but still others will have the large oval markings entirely covered 

 with a film of silver, indicating that there is a still finer and ordinarily 

 invisible "tertiary structure'' or plate, which serves as a basis on which 

 the silver is deposited. Exactly the same conditions may be noted on 

 the external plate of Coscinodiscus asteromphalus , but there is never an 

 indication that the eye-spots on the inner plate are anything but clear 

 openings. 



January 27. 

 The President, Samuel G. Dixon, M.D., in the Chair. 



Twenty-two persons present. 



The following were elected members : James Spear, Henry A. Lang, 

 and Edw. B. Meigs. 



William Morton Wheeler, of Austin, Tex., and Theodore Boveri, of 

 Wiirzburg, were elected correspondents. 



The following were ordered to be printed : 



