THE PRECIPITATION OF CALCIUM CARBONATE IN THE SEA. 499 



The colonies appeared to be of two kinds when grown on Peptone 

 Agar, one much more plentiful than the other. Subcultures made 

 from these colonies in Gran's medium showed that the bacteria 

 forming the most common type of colony produced an active denitri- 

 fication, while the others grew very slowly in this medium and 

 produced no denitrification. 



The characteristics of the denitrifying form are as follows : — 



The bacterium is a very minute, actively motile short rod, with 

 rounded ends, readily giving rise to involution forms in old cultures. 



On the Potassium malate, or Peptone Agar media, colonies are 

 visible as minute white specks after 6 to 8 hours, when the room 

 temperature averages 29"5° C. After about 18 hours the colonies 

 are well developed ; they are white in colour, circular, but with a finely 

 irregular outline, and have a granular appearance. Superficial colonies 

 are much elevated at first, but as growth proceeds spread rapidly over 

 the surface of the Agar. Deep colonies remain small, circular, and 

 discrete. 



Growth is somewhat more rapid on Peptone Agar than on the 

 Potassium malate Agar, and the older colonies develop a brownish 

 tinge in the centre when growing on the former medium. On 

 Gelatin Peptone {0°/^^ Peptone in sea- water and kept at between 

 20° and 25° C. to ensure the medium remaining solid) growth was 

 very slow : in stab cultures growth proceeded slowly from the surface 

 downwards, leaving a funnel-shaped depression of liquefied Gelatin. 



Acid formation, as shown by the Neutral Eed reaction, occurs in 

 Dextrose, Laevulose, Mannite, and Cane Sugar, but not in Lactose 

 media. 



Growth is inhibited at a temperature of 10° C, but takes place 

 slowly at 15° C. 



Growth is much retarded by exposure to bright sunlight, but the 

 bacteria are not killed by a ten hours' exposure. 



The bacteria are facultative anaerobes, but growth under anac'robic 

 conditions is very slow. 



In Gran's medium growth is rapid, but no growth occurs if the 

 Potassium nitrate be omitted, or if the Calcium malate be replaced by 

 Calcium carbonate. Growth in a pure solution of Peptone in sea- 

 water is slight, but becomes abundant if Potassium nitrate be added, 

 when denitrification quickly ensues. The most rapid early growth was 

 produced in sea- water containing 2°/^^ Peptone, l%o Potassium malate, 

 and 0"57oo Potassium nitrate, and in this clear medium a slight floccular 

 precipitate, presumably of Calcium salts derived from the sea-water, was 

 soon formed. Growth was also rapid at first in a solution of 57oo Potas- 



