1 88 Remarks on the Genus Aulacodisais Ehrb. [Sess. 



Argus, A. Thumii ; (11) Blanditi — subsection a — A. con- 

 cinnus, A. prominens, A. Ivittoni ; subsection y8 — A. Eattrayii, 

 A. sollittianus ; (12) Speciosi — A. negiectus, A. umbonatus, A. 

 lucidus, A. coronatus. 



Tbe most natural and sbarply circumscribed of tbese groups 

 are the Septati, Tenerrimi, Complanati, and Spectati. The 

 Areolati and Blanditi meet one another at A. radiosus and A. 

 concinnus ; the Inflati and Mirabiles at A. macraeanus and A. 

 archangelskianus ; the Speciosi and Spectati at A. umbonatus. 

 The entire series of forms may be regarded as springing from 

 the simplest form of the Complanati, namely, A. suspectus, and 

 through this directly from tlie Coscinodisci. 



The specimens connected by the bars in the annexed list 

 (see Table) are those that must be looked upon as most 

 nearly of equivalent value in the evolutionary grade. In 

 young frustules of A. affinis and A. margaritaceus, the modifi- 

 cations in the convexity and proximity of the markings may be 

 studied with considerable ease, but no mathematical law is 

 determinable by which one may predict what the progeny of a 

 given mother-frustule may be as regards the number of 

 primary rays, processes, or markings in a given space (say .01 

 mm.), since the opposite valves of a single frustule are found 

 to vary in these respects, and so all the more may valves 

 resulting from fissiparous division vary, and yet more those 

 that are the product of conjugation. 



The living frustules of this genus, though sometimes gathered, 

 remain undescribed, and in many instances quite unknown. 

 A wide field thus remains open for the fortunate observer who 

 chances to meet such forms. Young frustules, though found 

 occasionally, are for most of the species also still enshrouded 

 in darkness, and the getting at the history of the evolution of 

 their markings may be the reward of the careful worker in this 

 region. Fissiparous division is but inadequately understood, 

 and conjugation has never been seen. There is thus in this 

 small department still ample work to be accomplished, and the 

 same is applicable to most of the genera of this extensive 

 department of Botany. 



