160 THE ROYAL SOCIETY OF CANADA 



As the breadth of a given species may vary as its valval or zonal 

 aspect is presented, the letter v or z prefixed indicates the fact, thus: 

 v61(17) or z61(14). The valval breadth is always at right angles to 

 the zonal breadth. 



In the case of the tubular species like Melosira, the length of the 

 unit frustule is the diameter of the tube-like filament. The zonal 

 breadth is the length of the unit frustule from articulation to articu- 

 lation — in the direction of the filament. The valval breadth is the 

 diameter of the end view of the filament — in this case the diameter of 

 the circular valve, and is identical with the length of the frustule 

 although at right angles to it. 



When the filament has not a circular cross section, the longer 

 diameter of the valve is the same as its length, the shorter is the valval 

 breadth. For instance: in a band of connected frustules like Rhabdo- 

 nema or Tahellaria, the breadth of the ribbon is the length of the frus- 

 tule or unit diatom. The zonal breadths are the breadths of the in- 

 dividual frustules across the connecting zones tying them up into the 

 band or ribbon. The valval face will not be visible until a frustule 

 breaks ofif from the ribbon and turns on its longitudinal axis ninety 

 degrees. 



When the valval breadth varies symmetrically or distinctively, 

 for example, as in a variety of Navicula Crabro 36 microns long which 

 contracts midway to 8 microns and swells out towards each end until 

 it is 14 microns broad, the facts can be briefly noted thus: v36(14:8:14), 

 the colons separating indefinitely the maximal and minimal breadths. 

 The formula of Synedra Baileyi (MacKay) would be expressed thus : 

 v271 (9.5:7.5:8:7.5:9.5), which means that the valval face of the diatom 

 271 microns long is capitate, 9\ microns broad then narrowing down to 

 1\ and swelling up to 8 at the center, and so on. 



The fineness or coarseness of sculpture is another important fea- 

 ture which is capable of brief notation, whether they are lines, rows of 

 points, pearls or cell-like elements. The letter "s" at the end of the 

 parenthesis connotes the sculpture. The figure following indicates the 

 number of elements in the space of 10 microns, thus: Synedra Baileyi 

 might be described: v270(9:7:8:7:9) sll, the latter figure indicating 

 that the lines of sculpture are eleven in the space of 10 microns. 



Nitzschia socialis Greg, with its coarse sculpture on the keel and 

 its fine sculpture on the valve can be represented thus: vl27(8)s6:15, 

 the two orders of sculpture being separated by the colon. 



When the sculpture is of the same order but varies in fineness, 

 the figures are separated by a dash ( — ) beginning with the central 

 and ending with the peripheral lines or stiiœ, thus: Cocconeis sciitel- 

 lum Ehr. var. Baldjickiana Grun. = v50 (30)s6 — 4, the two last figures 



