270 K. M. NELSON AND G. C. KAItOP ON 



tions are large and irregular in shape around the margin, but 

 smaller and circular in the centre. A broken areolation is 

 figured to show the fracture passing through the perforations 

 (PI. XVII., Fig. 2). 



3. Triceratium favus. This diatom is very similar to the pre- 

 ceding. The coarse areolations are hexagonal in form and very 

 deep. At the bottom of these is a delicate perforated membrane, 

 the perforations being circular and arranged for the most part 

 in rows. Fig. 3 shows a fracture passing through the minute 

 perforations, the resolution of which may be considered one of 

 the most crucial tests for the microscope of the present day. 



4. Eupocliscus argus. This diatom differs from the above, 

 inasmuch as it possesses two separate membranes, one contain- 

 ing the coarse and the other the fine areolations. The outer is 

 a strong, coarsely-marked structure, the areolations being for 

 the most part circular or oval in outline. The intervening 

 silex is granulated on the exterior, and has a brownish colour 

 by transmitted light. With reflected light, however, it appears 

 white and sparkling, not unlike loaf-sugar. The interior 

 membrane is yevy transparent and covered with minute perfora- 

 tions (only lately discovered, and which have been called 

 tertiary markings). But in addition to these are what have 

 long been known as the secondary markings, viz., white bright 

 spots, which are arranged in row r s radiating from the centre. 

 These secondary markings must not be regarded as perforations, 

 as we have not found an instance of a fracture passing through 

 them. 



Fig. 4 shows the secondary and tertiary markings on the 

 interior membrane, as seen through the coarse areolations of 

 the exterior membrane. The best way of examining the 

 secondary markings is to use a f or T \ objective, with a 

 lieberkuhn, the specimen mounted dry, with the concave side 

 uppermost. The tertiary are more difficult to see, and will 

 require a higher power. 



Fig. 5 shows the fracture passing through the perforations in 

 a valve of Pleurosigma angulatum. This diatom has but one 

 membrane, and only one kind of perforations. To show this 

 properly a lens must be very well corrected, and have its 

 glasses very perfectly centered. 



