T. F. SMITH ON THE VALVE OF PLEUROSIGMA. 305 



long triangular piece by the side of the median line. At that 

 particular place, in the actual specimen, the valve is cracked 

 half across, and a part has sunk down, leaving the piece of 

 delicate structure adhering to the cover. Some gentlemen are 

 here to-night who have examined the specimen under the micro- 

 scope, and can prove that all the valve was there. 



Here, then, I think, is sufficient evidence to prove that each 

 valve of Pleurosigma formosum is built up, not of one but, of 

 several layers of structure, and I for myself should feel no hesi- 

 tation in asserting, from analogy alone, that all the valves of the 

 different species of Pleurosigma possess at least a double struc- 

 ture. I have not trusted to analogy, however, but have been 

 fortunate enough to discover, by direct evidence, that the valves 

 of five other species at least have a compound structure, viz., 

 P. decorum, P. balticum, P. angulatum, and two species from 

 Richmond, Virginia. I hope by-and-bye to be able to demon- 

 strate compound structure in the other species, but at present 

 am waiting for materials. 



It is not my intention to go minutely into the structure of 

 each particular species, as the photographs will somewhat 

 explain themselves ; but I wish to dwell a little on that of 

 Pleurosigma angulatum as being the diatom around which 

 most discussion has arisen. In treating of this diatom an initial 

 mistake has been made, by both schools, in assuming the struc- 

 ture to be regular, when in truth nothing can be more irregular 

 than the so-called markings. Dr. Anthony called attention to 

 this in the 5th Vol. of the Monthly Microscopical Journal, and 

 figures the different appearances it presents at the ends and 

 middle of the valve. The drawing is a very fine one, and as an 

 interpretation of the image has but one fault — that it is all 

 wrong. In his drawing the " beads " go close up to, and butt 

 against the median line ; but in reality a row of perforations 

 larger than the others runs parallel to it on each side, and a 

 similar row along each outside margin of the valve. Besides 

 this, on almost every valve are parts where the partitions 

 between the perforations are absent — nearly always at the ends, 

 and often in the middle — and it is this peculiarity which has 

 enabled me to determine the existence of a double structure on 

 this diatom. At one focus you will see the " markings " 

 regular, and then, by merely raising or lowering the focus, its 



