Dr. G. C. Wallich on an undescribed Form 0/ Amoeba. 365 



different portions of the Diatom-valve under the action of fluoric 

 acid — assuming, as every portion is of precisely similar density, 

 that the thinner parts must be the first to succumb to the 

 eroding action. I may mention that the acid was very gradually 

 applied in fumes to the Diatoms previously dried and fixed on 

 slips of mica. 



In the stouter and more coarsely marked Navicula, in which 

 the median line is surrounded by a broad unstriated area of silex, 

 from the outer margin of which the striation commences, the 

 striated portion was invariably destroyed first, the dotted points 

 being the foremost to yield. The median line and its adjoining 

 area yielded last. 



In Cocconeis distans the dots at once yielded, and then increased 

 in size until the intervening portions of silex were altogether 

 eroded. 



In Biddulphia, Triceratium, and Isthmia, the interspaces yielded 

 first. The costae of B. pulchella and B. regina yielded last. 



In Pleurosigma formosum and P. halticum the action on the 

 entire structure was so instantaneous and complete that it was 

 difficult to apply the smallest quantity of the fumes without 

 altogether consuming the valves. But wherever portions re- 

 mained, the outline was identical with that shown when the 

 valves ai-e fractured, thus clearly confirming the view that the 

 portions constituting the linear markings are the thinnest. In 

 these forms the median and terminal nodules were the last to 

 succumb. 



Lastly, it is worthy of notice that in those portions of the 

 Diatom-valve which present a delicate pinkish tint the siliceous 

 structure is the thickest. Where the tint is grey or inclining 

 to green, the film may be regarded as being of extreme tenuity. 

 The same feature holds good amongst the Polycystina, and is 

 indeed more strikingly observable in that family, owing to their 

 superior size and solidity. 



XLI. — Further Observations on an undescribed indigenous Amceha, 

 with Notices on remarkable forms of Actinophrys and Difflugia. 

 By G. C. Wallich, M.D., F.L.S., F.G.S., &c. 



[Plate IX.] 



The very singular characters presented by the new form of 

 Amoeba, to which attention was drawn by me in the last Number 

 of ' The Annals,' having induced me to keep it under constant 

 supervision during the bygone month, I am glad to be enabled 

 to confirm the description there given, and at the same time to 

 add several new facts regarding it and two other species of in- 



