18 THK NAUTILUS. 



squarely cut off, the anterior broadly rounded. The surface is 

 smooth and shining with very fine lines of growth. The color 

 is light yellowish green or greenish horn. It occurs in clear 

 fresh-water ponds or the sheltered parts of rivers, usually em- 

 bedded in soft sticky nmd, the siphons protruding just above 

 the .surface. Very often it may be seen climbing about on sub- 

 merged vegetation. The seasons of greatest apparent abundance 

 are the spring and early summer months. It quite frequently 

 occurs in ponds which are dry throughout the greater part of the 

 year. 



Spfiaeriuiii simile is one of the largest species of the family. 

 An adult specimen may be eighteen millimeters long and thir- 

 teen millimeters high. The shell is rather solid, almost equila- 

 teral, transversely oval (Fig. 10), the anterior and posterior 

 margins almost equal. The_surface is shining, pale green, with 

 coarse growth-lines in young specimens but in older ones it is 

 dull, dark brown to black with 9nly those growth-lines evident 

 which mark the ends of gro^vjtb. periods. It prefers clear, cold 

 streams but may be found in quiet pools of rivers and lakes. 

 It buries itself in soft mud or debris usually an inch or more 

 below the surface. It communicates with the water above 

 through a small hole in the mud, I have no record of this 

 form occurring in places which are not well supplied with water 

 throughout the year. 



Methods. During the early part of the work, expanded 

 animals were fixed in hot water or hot mercuric chloride. 

 This method had one great advantage, in that it allowed very 

 litte contraction of organs. But for cell structure the following 

 was found to be better. The animals were prevented from 

 completely closing the shell by a small piece of wood inserted 

 between the valves. In this condition they were placed in cold 

 saturated mercuric chloride with two to three percent of glacial 

 acetic acid. They were kept in the fixer for twenty-four hours. 

 The acidity of the fixer removed nearly all of the calcium salts 

 of the shell. The remainder was removed by one half to one 

 percent of hydrochloric acid in sixty-seven percent alcohol. 

 The entire animal was imbedded in paraffin. Serial sections 

 were cut from six to ten microns in thickness. Delafield's 



