440 THE MICROSCOPE. 



ways at the long and short diameters. On the margin 

 there are two series of dots, sometimes joined ; and on the 

 oval centre there are also dots about the margin, while the 

 middle is nearly plain. (Fig. 225, No. 7.) Actinocyclus has 

 a round bivalve flinty case, with numerous cells formed by- 

 radiating partitions; very often every alternate cell only 

 is on the same plane. The specimen in the Museum of 

 the College of Surgeons is exquisite in its markings ; it 

 was found in some Bermuda earth, and has a beautifully- 

 raised margin, and a five-rayed star in the centre ; the 

 number of cells is ten, five being on one plane and five on 

 another. One set has the usual hexagonal reticulations 

 crossed with diagonal lines, the other has the same lines, 

 with a much smaller series of triangular reticulations, so 

 disposed that they appear to form with each other parts 

 of very small circles. One valve from this specimen is 

 represented in fig. 224, No. 2. 



As well as the beautiful shell of the Coscinodiscus, 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. (Fig. 224, No. 1.) This 

 is found in the guano of Ichaboe, and also in earth from 

 the United States, as w^ell as among sea- weed from Japan, 

 and the Cape of Good Hope. Mr. Shadbolt believes : 

 " These shells are not, strictly speaking, bivalves, although 

 capable of being separated into two corresponding por- 

 tions ; but are more properly multivalves, each shell con- 

 sisting of two discoid portions, and two annular valves 

 exactly similar respectively to one another." (See Micro- 

 scopical Society's Transactions, for an excellent paper on 

 these shells by Mr. Shadbolt.) 



Artists who design for art-manufacturers might derive 

 many useful hints from the revelations of the microscope, 

 as evidenced in the arrangement of the shell last noticed, 

 and in that of the genus Coscinodiscus; a very handsome ob- 

 ject, the shells of which are marked with a network of cells 

 in a hexagonal form, arranged in radiating lines or circles, 

 and varying 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 ; 



