170 THE MINUTE. 



active genus,* wliicli, being wholly without the foot com- 

 mon to this class of animals, is always found swimming, 

 being ap^^arently incapable of resting, or, at least, of 

 crawling. The group contains many species, and most of 

 them have their shells ornamented with some symmetrical 

 variation of the surface. In one,t a ridge runs down the 

 middle of the back, dividing the shell into two equal 

 lateral portions, each of which is subdivided into about 

 ten polyhedral areas by intervening ridges, of which no 

 two are alike in form, though each corresponds accurately 

 with its fellov/ on the opposite side. The form of each 

 area is constant in every individual. In another,]: the 

 medial line is occupied by five areas, of which the first is an 

 imperfect hexagon, the second is square, and the posterior 

 three are hexagons ; from the salient angles, other ridges 

 run off sidewise, and form other imperfect polygons. In 

 others, § the division is into many hexagonal tesselations, 

 varied with other forms in the outer or hinder areas 

 according to the species, and having the peculiarity that 

 the dividing ridges are well-defined narrow elevations 

 armed throughout with conical points in single row. 



I may be accused of exaggeration in presuming all these 

 creatures to be seen in one drop of water. I do not pre- 

 tend to be depicting them from one single actual observa- 

 tion ; at the same time I may say that I have described 

 nothing but what I have personally observed ; and I 

 have known many small pools and other collections of 



* Anurozcti. + A. tecta. X A. curvicornis. 



§ A. acidcata, sarulata, &c. 



