42 E. M. NELSON AND G. E. KAROr 



our disposal, and for his assistance in naming the specimens. We 

 are indebted to him for the specimens figured 1, 2, 5, 7, 8, 9. 



1. Coscinodiscus centralis exhibits a very beautiful structure. 

 The bars of the so-called hexagonal, or coarse structure, which are 

 more delicate than usual, are seen lying over the finer perforated 

 membrane. In some instances a bar of the coarse structure 

 bridges over a perforation in the lower structure. Average 

 diameter of coarse areolation '0000957 inch. 



2. Bacteriastrum varians, showing fine, thorn-like points 

 arranged spirally round the ray. The length of the thorn is 

 TooV'oo i ncn > assuming it to be one-tenth of the diameter of the 

 ray. 



3. Euphyllodium spatlmlatum. The coarse areolations are filled 

 up with a structure of very unusual form, some of the minuter 

 perforations reminding one of telescopic glimpse-objects. Diameter 

 of coarse areolation, '0000904 inch. 



4. Arachnoid iscus Ehrenbergii, showing points projecting into 

 the areolation from its edge ; some varieties of this diatom have a 

 more elaborate pattern. Diameter of areolation, *000117 inch. 



5. Stalk of Kittonia elaborata, exhibiting a delicate perforated 

 membrane, covering the bell-shaped end of the pipe. This curious 

 appendage resembles in miniature the rose of a watering-pot. 

 Major axis, '00135 inch. The tube-like processes or feet at the 

 angles of the triceratia have minutely perforated caps to them. 

 Those on Triceratium fimbriatum are not so easy to resolve. 



6. Isthmia nervosa, described above. Major axis, '000274 inch. 

 There is a very minutely perforated membrane in the smaller 

 areolations on the hoop of this diatom — a test. 



7. Fragment from Briinn deposit. A very beautiful but un- 

 usual pattern, which we have named the fiddle-pattern, on 

 account of its resemblance to the / marks on a fiddle. Major 

 axis, -000202 inch. 



8. Fragment from Briinn. Major axis, '000283 inch. 



9. Aulacodiscus Slurtii, showing a secondary pattern outside the 

 primary. In all the examples we have hitherto brought before 

 you the secondary structure has been inside the primary. This 

 diatom, however, in common with several of the Aulacodisci, has 

 its secondary pattern exterior to its primary. Diameter of 

 primary structure, '0000478 inch. 



10. Asterolampra vulgaris. The well-known fine lacework of 



