162 THE MICROSCOPE. 



Artists who design for art-manufacturers might derive many use- 

 ful hints from the revelations of the microscope, as evidenced in the 

 arrangement of the shell last noticed, and in that of the genus Cos- 

 cinodiscus, another handsome object; the shells are marked with a 

 network of cells in a hexagonal form, arranged in radiating lines or 

 circles ; they vary from l-200th to l-800th of an inch in diameter. 

 A specimen found in Bermuda earth has on one of its valves two 

 parallel rows of oval cells that form a kind of cross ; they are 

 gradually larger from the centre to the margin. The angles of the 

 cross are filled with the hexagonal cells previously noticed. (Plate II. 

 No. 4.) 



The unskilled manipulator may for some time endeavour to adjust 

 a slide, having a piece of glass exposed not larger in size than a pea, 

 on which he is informed an invisible object worthy his attention is 

 fixed, before he is rewarded by a sight of Triceratium favus, extracted 

 from the mud of the too-muddy Thames. The hexagonal markings or 

 cells are beautiful, and at each corner there is a curved projecting horn 

 or foot. (Plate II. fig. 10.) In Bermuda earth there is a small species 

 found, which has its three margins curved ; and also a curious species, 

 which resembles a triradiate spiculum of sponge. 



It is remarkable how, in these minute and obscure organisms, we 

 find ourselves met by the same difficulties concerning any positive laws 

 governing the formation of any generic types, as in the larger and more 

 complex forms of animal and vegetable life. It appears as if we 

 could carry our real knowledge little beyond that of species ; and when 

 we attempt to define kinds and groups, we are encountered on every 

 side by forms which set at nought our definitions. 



Zygoceros rhombus is in three parts ; one central, which is like a 

 broad band, the others lateral, of a rhomboidal shape, and curved; 

 while at each corner is a projecting piece like a spine. It is en- 

 tirely marked with very small dots. (Plate II. No. 6.) 



As well as the beautiful shell of the Cosdnodiscus, found both in a 

 fossil and recent state, there is one of exquisite elegance and richness 

 of the genus Arachnoidiscus, so named from the resemblance of the 

 markings of the shell to the slender fibres of a spider's web. (Plate II. 

 No. 1.) This is met with in the guano of Ichaboe; it is also found in 

 the United States, as well as among the sea-weed from Japan, and the 

 algse of the Cape of Good Hope. Mr. Shadbolt says : " These shells 

 are not, strictly speaking, bivalves, although capable of being separated 

 into two corresponding portions ; but are more properly multivalves, 

 each shell consisting of two discoid portions, and two annular valves 



