420 THE MICROSCOPE. 



gard to the beautiful Surirella cvnstricta, the side view is 

 no longer bacillar, but the breadth of the valve is very 

 considerable, and when about to undergo subdivision it 

 becomes square-shaped. The distinctive character, how- 

 ever, of this genus, in addition to the presence of canaliculi, 

 is derived from the longitudinal line down the centre of 

 each valve, and the prolongation of the margins into 

 " alae." The sudden change in appearance presented to 

 the eye as the frustule is seen to roll over, is very remark- 

 able. As a rule, therefore, we must examine all specimens 

 in every aspect, to accomplish which very shallow cells 

 should be selected, say of 1-1 00th of an inch deep, and 

 covered with glass l-250th of an inch thick. A good 

 penetrating objective must be used, and careful illumina- 

 tion obtained. The examination of living specimens 

 should be conducted during very bright weather, the 

 mirror directed towards a white cloud, or even sunlight : 

 with coloured glasses to protect the eyes from injury. 

 The Diatomaceae are perhaps more widely distributed than 

 any other class of infusorial life ; they inhabit fresh, salt, 

 and brackish water ; many grow attached to other bodies 

 by a stalk (Plate II. No. 33), Licmophora and Achnanthes; 

 while others, as the Pleurosigma, No. 40, swim about 

 perfectly free in the water. 



There are a considerable number of Diatomaceae which, 

 while in the young state, are enclosed in a muco-gelatinous 

 sheath ; while others are attached by a stipes or stalk to 

 Algae. Ehrenberg recognised a tribe of compound Dia- 

 toms, with a double lorica, and introduced them into his 

 great family of JBacillaria, under the name of Lacernata or 

 Navicula. Silex enters largely into the composition of 

 their valves, but, being in combination with organic sub- 

 stances, it does not depolarize light. In several genera 

 silex is very deficient, and the wall of the frustule of 

 great delicacy. Mr. Brightwell, speaking of the lorica or 

 siliceous covering of the Triceratium, states " that the 

 valves are resolvable into several distinct layers of silex, 

 dividing like thin divisions of talc, and frequently of such 

 exquisite delicacy as to be difficult of detection." Na'geli 

 speaks of a mucilaginous pellicle on the inside of the 

 organic layer as a sort of third tunic ; and, as Meneghini 



