4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



empty, "dead" cells (unpigmented frustules), which is the only con- 

 dition in which they could be examined morphologically at all ade- 

 quately. In quite old cultures, empty frustules, and occasionally sepa- 

 rate valves, become more frequent. Empty frustules or valves are 

 dim-whitish in appearance and almost invisible in water. This "whit- 

 ish" appearance of the diatom in water under ordinary full-field 

 illumination suggests the advantage of "dark-field" illumination and, 

 indeed, the latter (or "phase-contrast") is a good way to bring out 

 more prominently the obscure cell features. The diatoms are most 

 readily located by the much greater visibility, in girdle view, of the 

 false "central nodule" which can be picked up as a bright triangular 

 spot, from which the rest of the cell outline can then be followed. 

 Were it not for this the shells would not be easy to make out or would 

 be overlooked completely. In valve aspect the raphe and transapical 

 groove are the more easily seen features, appearing as moderately 

 bright white lines. 



Although this diatom is necessarily described on fewer structural 

 features than usual, it is felt that it should be readily identifiable from 

 these features, and by the very characteristic "tear-" or "pear-shaped," 

 alternately arranged chromatophores, which afford it a rather con- 

 spicuous and distinctive character. I have not been able to secure a 

 separate view of the inferior valve, and so that has been hypothecated 

 from the girdle view of the whole frustule. 



There is dif^culty and uncertainty in making out even the generic 

 status, although the diatom character is immediately evident and not 

 at all to be questioned. The prevalence, range, and frequency of 

 Achnanthes subhyalina are not likely soon to be determined. Its small 

 size, frailty, and general obscurity make it a form not likely to be 

 found by the conventional methods of examination of natural mate- 

 rials that account for the discovery of most species of diatoms. It is 

 unlikely to be found except when in quantity in isolated cultures, 

 which suggests that there may well be many other such diminutive 

 forms that have escaped notice due to the limitations of conventional 

 procedures. On the other hand, the readiness and rapidity with which 

 it grows and its evident hardiness suggest that it may be a widely 

 distributed and abundant species. Because of its frailty and low 

 degree of silicification, the shells are not likely to persist after death 

 in the natural environment or to be recognized if they do persist. It 

 must be observed in the living state for determination or recognized 

 from dead shells in culture material. By present methods no perma- 

 nent preserved "type" preparations, such as microscope slides, have 



