NATURAL HISTORY OF THE DIATOMACEiE. 43 1 



are extremely graceful in sculpture. In form it is discoid, on what is 

 known as the "side view," but, unlike any of those we have seen so far, 

 the surface of the valve is divided into segments which radiate from the 

 centre, and are arranged alternately elevated and depressed, so that on a 

 front view the frustule appears undulate. When we look straight down 

 upon the valve, it has very much the appearance of a wheel. In some 

 species the markings of the raised segments are different from those on 

 the sunk portions, while others have the markings of the same character 

 all over the valve. They are, however, always of the same general char- 

 acter as those described as occurring in Aulacodiscus. This is likewise 

 a marine genus, and some of the most beautiful species belonging to it 

 have been found as yet only in the fossil state. 



There are many other discoid forms which we cannot stop to describe, 

 but must pass on to consider some other genera. 



Nearly allied to the true discoid diatoms, and in fact having a few cir- 

 cular species, are two genera which it will be well to describe here. One 

 of them is called Biddulphia, and is found only in salt water, although 

 one species was seen by the late Prof. Bailey in the Hudson river at 

 West Point, where the water is not all salt ; but, strange to say, the tide 

 reaching up as far as this, the salt water creeps up under the fresh, so 

 that at this point salt and fresh water forms of vegetation appear along- 

 side of each other. Biddulphia grows in chains attached to submerged 

 objects, more commonly the larger plants. It has valves either orbicular, 

 elliptic, or more or less pointed in two directions, and approaching in 

 outline to the boat-shaped genera to be presently described. In fact, 

 the outline of the valve in Biddulphia varies very greatly, as is seen by 

 the figures given. At two opposite points on the valve are projections 

 upwards very much like the feet of Aulacodiscus, and, in fact, they may 

 be considered their analogues. So when Biddulphia is looked at on a 

 front view, it looks like a number of little wool-sacks ; and the species 

 which Prof. Bailey found at West Point, and which is not very uncommon 

 along the Atlantic coast of the United States, has very much that appear- 

 ance, especially as the frustules grow in the form of a chain, with these 

 projecting portions united, often alternately, so that the chain becomes 

 of a zigzag form. Sometimes the surface of the valve also bears upon 

 it certain spines, varying in number in different species, and usually 



