14 PROCEEDINGS OP THE ACADEMY OF 



The valves which are singularly light and graceful, the linear striation giving 

 the surface much the aspect of a scale of Lepisma, are rarely found united by 

 the connecting membrane. Mr. Greenleaf communicates the following facts, 

 with relation to this diatom; " The longitudinal lines are parallel throughout, 

 gradually fading away, so that they are not seen near the apices ; they are 

 faintly visible with careful illumination and focussing over the stauros ; 

 margin beaded." I have not been able to verify Mr. G.'s observations. It 

 seems to me, that the longitudinal striae are internal, and underlie the stauros, 

 and that they are traceable up to the apex, in that limited number of parallel 

 stria; which the narrow area of the extremity can accommodate. The beaded 

 marginal appearance, is probably due to the effect produced by the inflexion 

 of the striae. Mr. Stodder believes the longitudinal striae to be corrugations 

 of the internal membrane, designed io strengthen the valve. 



The figure (fig. 6, pi. ii.) is not sufficiently elongate for an average speci- 

 men of this species. 



" Tetracyclic" (abnormal) ? (pi. ii., fig. 3a, & b.) F. V., much as iu the 

 typical form, (genus) ; filament with a central constriction ; frustules small ; 

 septa alternate, equal ; V. deeply constricted in the centre, (not unlike a 

 dumb-bell crystal of oxalate of lime). 



Hab. of Tabellaria and Tetracyclus. Lafayette Mt. Lake. 



This singular form occurs only at the above locality. Being minute and 

 much intermixed with Tabellaria and Odontidium tabellaria, it is apt to be 

 overlooked on a crowded slide. The septa are quite insignificant. They 

 exist at both ends of the valve. This anomalous form suggests a metamorphic 

 variation, mediate between Tabellaria and Tetracyclus, and is a true comprehen- 

 sive type, although less perfect than Surirella intermedia. The relations of the 

 septa, small as they are to the valve, and to each other, are constantly those 

 of Tetracyclus, (never of Tabellaria), though the impressible character of 

 outline, passively yielding to the disturbing force, has wandered far away 

 from the typical pattern of that genus.* 



I will now, as briefly as possible, sum up the conclusions I have tried to 

 establish in this and the previous paper. After which summary will be 

 found a comparative table of some of these species. 



These are (1). That the genera Syncdra, Nitzschia, Tabellaria, and perhaps, 

 Odontidium and Himantidium, made their first appearance as prevalent forms 

 on this continent, at varying epochs, since the Glacial period. 



(2). That there are epochs, which may be termed transitionary, coinciding 

 with the abundant introduction of genera, (as above), marked by a singular 

 relaxation of the laws which govern generic character, (melainorphic force.) 



(3). This metamorphic force while respecting the more fixed and positive 

 generic characters, {essential), usually attacks the more variable and unimpor- 

 tant characters, (i. e. those common to most diatoms), {non-essential), of those 

 genera most nearly allied to the incoming one, giving rise to comprehensive or 

 synthetic forms ; as Surirella intermedia, Sanceps Sec. 



(4). During these epochs, there would also seem to exist a subjective 

 variation in many of the cotemporary genera, whether allied or not to the 

 prevailing one originating in this objective force still affecting the non-essen- 

 tial characters, principally form and outline, e. g. objective genera Synedra, 



*(The metamorphic force seems to run riot in this particular locality, (Mt. Lafayette Lake), and 

 nearly all the species appear to participate in this tendency to depart from their normal type. The 

 largest and showiest frustules of the pointed elliptic, which is by far the finest variety of N. 

 firma, pass down through many intermediate forms into a minute and characterless N. affiriie, 

 N. rhnmboides ranges from tho smallest apiculate, up to the magnificent var. b. (pi. ii., fig. 11), 

 curious and anomalous varieties of Tabellaria, of Odontidium tabellaria, some punctate, others 

 Synedroid, eccentric varieties of Synedra hemicyclus, of Eunotia incisa, Eu. camelus, Eu. pentagly- 

 phis, Eu. robusta, of Himantidium gractU, concurring with the abnormal ,Actinella punctata, 

 Surirella intermedia, and & delicatissima, altogether form a grouping confused and incoherent 

 beyond precedent.) 



[Jan. 



