344 



PROCEEDINGS OP THE ACADEMY OP 



acter; and recent gatherings, after a period of three years, show no differ- 

 ence in this respect.* 



The peculiar notched inflation, which appears to be a fixed character of 

 this diatom, equally with the tendency to punctate arrangement observable 

 in several species in the gathering, possesses a certain significance, as show- 

 ing how strongly a dominant Nitzschoid {Synedroid) leaning exists throughout 

 the group. I have before alluded to the disposition to marginal punctula- 

 tion evident in A. punctata, Tabellaria fiocculosa, and one or two doubtful 

 smaller forms in the Saco mud, as affording probable illustration of a sympa- 

 thetic intrinsic force, tending towards the Nitzschoid type. In A. punctata 

 there would seem be, as it were, a double exercise of this attractive force, 

 conducting, not an allied form, but one only remotely connected with it, to- 

 wards the same type. The antetype in the present case would appear to be 

 Eunotia, between which and the Synedroid genera this anomalous form con- 

 stitutes an intermediate or comprehensive link. 



Eunotia (monodon to polyodon) is the prevailing form in the gathering. It 

 occurs with every peculiarity of dorsal prominence, from a faintly percepti- 

 ble undulation to the sharpest and most serrate crenature, a fact singularly 

 opposed to the experience of the late Prof. Gregory, who limited this ten- 

 dency to vary to the two species, E. bigibba and triodon. Now this disposi- 

 tion to extreme variation, even in non-essential characters like the above, in 

 species not ordinarily variable, concurring with a comprehensive type like 

 A. punctata, induces me to think that these varieties of Eunotia, illustrate a 

 series of successive approaches to the Nitzschoid type, which has culminated 

 in the case of E. polyodon in the abnormal and irregular genus I am de- 

 scribing. This, had it not been for the peculiar mode of its growth and aggre- 

 gation in stellar groupings, would have ranked as an aberrant Eunotia ; and, 

 in fact, before meeting with the living form, I had distributed specimens un- 

 der the name of Eunotia fibula. It has the arcuate form, terminal nodules 

 and convergent striation, which usually characterizes that genus, and pre- 

 sents some curious points of analogy to the prevailing species, i?. polyodon, in 

 respect to the resemblance between the notched inflation of A. punctata, and 

 the two terminal undulations of the valve in the former diatom. The ventral 

 or concave aspect of both is similar, or nearly so, and the terminal nodules 

 and striation do not materially differ. By suppressing all the dorsal eleva- 

 tions, excepting the terminal one at one end of E. polyodon, and attenuating 

 the valve, a form in outline nearly identical with A. punctata is obtained. In 

 suggesting this resemblance, I do not wish to imply that Actinella has re- 

 sulted by progression or degradation from this or any other species of Euno- 

 tia, but merely to infer the possibility that, in accordance with the laws 

 which govern the comprehensive type, the non-essential character of dorsal 

 crenulation common to the latter genus and its ally Himantidium, is becom- 

 ing merged into the smooth lines of the Synedroid genera.f 



To Asterionella this form presents very few points of resemblance. The mode 

 of growth is Bomewhat similar, but the frustules are essentially different. 

 Both, it is true, are bacillar forms, and possess unequally developed ends ; 

 but the valve of Asterionella is straight and symmetrical, while that of Acti- 

 nella is curved and unequally bifid at the larger end, which is free, and not, as 

 in case of the former genus, attached to its fellows by the adjacent angles. 



* In this connection and in corroboration of the view before advanced, I may state that the 

 Wolfboro' mud was taken from a stream whoBe bottom has relations to the valley drift very 

 similar to those of the Saco deposit. Besides A. punctata I have been able to identify thre* 

 of the Saco n. sp. in this mud, viz., a degenerate variety of Surirella intermedia S. deliea- 

 tissima with generic characters still more repressed and a third form too imperfectly charac- 

 terized to admit of present classification. 



+ A further illustration of this bias towards the Synedroid type is afforded in the case of Himan- 

 tidium qracile, in this gathering, whose frustules are in many cases so attenuated and dehiscent 

 to render it difficult, if not impossible, to distinguish them from those of Synedra. 



[Deo. 



