52 THE VEGETABLE KINGDOM. 



minute cells forms a hoop round the valves, and when they 

 multiply the young cells slip out from between the valves, 

 and the hoop often becomes detached. 



Amphitetras is a square cellular diatom, which frequently 

 has its frustules piled up one over the other, with a large 

 cell in each corner of the frustules. They are found alive 

 in the sea off Cuba and the Canary Islands, fossil in 

 Bermuda earth and Barbadoes deposit. 



ISTHMIA ENERVTS. 



A lovely diatom, found on seaweed on the English coast 

 and in the Channel Islands. Its exquisite areolated struc- 

 ture is very remarkable, and will repay careful examination. 

 Its mode of increase is unlike all others. Two cells form 

 within the valves, and as they enlarge break forth ; but still 

 the siliceous hoop which once joined the new frustules to 

 the old one remains attached for a time round one of them 

 and alters its shape, causing some to appear truncated 

 instead of round. The areolse of Isthmia are never well 

 seen except with the parabolic illuminator, or mounted dry 

 and viewed with a binocular microscope. 



ARACHNOIDI8CUS. 

 From arachne (a spider), and discus. 



This beautiful disc is one from the guano, and is also found 

 attached to seaweed ; especially one species, which is much 

 used by the Japanese in making soup. It does, indeed, 

 somewhat resemble a spider's web. But how can we 

 describe the wonderful delicacy of its tracery, or cease to 

 wonder at the perfection of its form, when we learn that 

 this double disc has two inner valves ; the outer one horny, 

 upon which are the web-like marks, is indestructible in 

 nitric acid ; and the inner valve siliceous, supports the upper 

 one upon fretwork like a gothic window. This should 

 always be looked at with a Lieberkuhn, or a parabolic 

 illuminator. 



