306 H. MORLAND ON DIATOM STRUCTURE. 



is evident on examination ' of broken valves. This thin layer is 

 strengthened, unlike certain of the Coscinodisci, Triceratice, and 

 Aulacodisci, on the interior instead of the exterior surface, by 

 means of radial plates set edgewise from a central ring to the 

 circumference, and these j)lates themselves are strengthened on the 

 inner edge by its being formed somewhat like an ordinary railway 

 rail. The ends of the plates widen considerably, and join each 

 other with graceful curves, uniting also with the inner ring and outer 

 circumference. The interior plates are still further strengthened 

 by being joined to the outer layer by little brackets, placed either 

 between the perforations or on the inner side of the circumference. 

 There are also shorter radial plates of simple construction springing 

 about half way between the central ring and the circumference, and, 

 joining the outer border. I have endeavoured to explain this 

 structure as clearly as I can, but I feel I have not done it justice. 

 All I can say is, that the more clearly I began to understand the 

 structure of this diatom, the more was I lost in admiration. 



Amongst my diatomaceous material I have some from Tampa 

 Bay, Florida, which contains a number of Evpodiscus Argus, 

 having the outer substance of the valve very much corroded. Look- 

 ing at ordinary valves of this diatom, it is very difficult to make 

 out what its real structure is, the outer portion being dense, with 

 irregularly-shaped openings all over it ; but in the Tampa Bay 

 forms this outer substance is entirely wanting in the central part, 

 gradually becoming perfect towards the circumference. An ex- 

 amination .shows that the inner substance, or plate, has minute 

 markings radiating from the centre ; above this is an hexagonal 

 structure (somewhat resembling that of Triceratium favus), but 

 which is wanting in the centre. In ordinary forms it would seem 

 as if there were a redundancy of silex, which granulated on 

 and around the hexagonal framework, which covers the outer 

 surface of the valve. Not long since I promised a correspondent 

 a slide of this form from the Tampa Bay material, and one day I 

 selected a quantity for mounting on a future occasion. When, how- 

 ever, I came to mount them, I found that a number of them had 

 got smashed (the very best ones, of course; "it is always so") ; 

 certainly there were some left, but they were not so good as I could 

 have wished. I did not care to search for more, knowing that the 

 really good valves required some looking for, so I had reluctantly 

 to make up my mind to send my correspondent a slide of the 



