342 PROCEEDINGS OP THE ACADEMY OF 



from the west and north, the Diatomaceae found in all these being principally 

 such genera and species as are found in most northern localities. I may add 

 that the Saco stream, at a short distance from its point of emergence from 

 the marsh, contains none of the new species. From these facts it seems 

 reasonable to infer that the local conditions which favor the development of 

 these are confined to a circumscribed area. Further, that these conditions 

 do not depend on any peculiarity in the vegetable growth which lines the 

 margin and extends for a short distance into the pond, is highly probable, as 

 this seems to be in all respects similar to that surrounding the streams and 

 ponds adjoining ; nor can it be said that there is any characteristic peculiar- 

 ity of the surface soil serving to distinguish it from that elsewhere found in 

 the valley bottom, in which I have vainly searched for the silicious remains of 

 these organisms. 



These facts of exclusion, in my opinion, go far to prove that the causes 

 which have perpetuated these varieties depend in no small degree on the 

 mineral constitution of the springs up-flowing from under the valley " muck" 

 or till, of which I am assured only a superficial covering overlies the boulder 

 drift at this point, and also, perhaps, to telluric impressions derived indi- 

 rectly from other sources. 



In further corroboration of this view, I refer to the presence in the gather- 

 ing of the living Eunotia hemicyclus, a species which has heretofore only oc- 

 curred in fossil deposits, of Eunotia incisa, also a fossil form, and to the 

 greater abundance and larger size of the siliceous valves of the new species 

 at from two to five inches below the surface mud. This is particularly the 

 case with regard to Surirella anceps, next to be described, the remains of 

 which are rarely met with in any quantity at a less distance than two inches 

 below the pond bottom. 



I shall reserve for future consideration whatever corroborative evidence of 

 the long period of time required in the production of these forms is afforded 

 by the tendency to extreme variation manifest in nearly all the species, and 

 by the disposition shown by some of them to assume a punctate arrange- 

 ment, such as characterizes Actinella punctata, and to a less marked degree Ta- 

 bellaria in the deposit. 



III. Surirella anceps, n. sp. The species now to be noticed I have re- 

 ferred to Surirella, not without hesitation. Frustules free, linear, F. V., 

 straight or very slightly sigmoid. V. Sigmoid, elongated with rounded sub- 

 acute extremities (in outline singularly vermiform.) Alae very small (some- 

 times nearly obsolete) or submarginal : canaliculi very inconspicuous, often 

 wanting: strive, very sharp and clear, extending to the wide and well de- 

 fined central space which runs from end to end of the V. [Over the inner 

 aspect (?) of the valve the striae appear to be continuous, or this phenomenon 

 may be exceptional. I have frequently met with valves in which the strife were 

 uninterrupted and have been unable to focus anythingresembling a central line 

 or even traces of alas, and this fact has induced me to thiDk that the thick- 

 ness of the substance of the valve may have prevented my doing so.] Color 

 of dry frustule, rich chesnut-brown. 



This species is quite as common as the preceding, although not as a living 

 organism. It occurs most abundantly in the mud which envelopes the roots 

 of the grass and plants at the northern end of the pond, and appears to be 

 an older species than S. intermedia, and I may add, a stronger one, as not- 

 withstanding the remarkable subordination of generic character and the ev- 

 idence of greater degradation from its type visible, it is less variable, both 

 in size, outline, and striation than this latter. Fig. 3, a and b. 



The slight and imperfect development of the alas and canaliculi in this 

 species requires a careful management of light to enable their being viewed 

 satisfactorily. They are best seen by using a condenser of moderate angular 

 aperture. 



[Dec. 



