NATURAL SCIENCES OF PHILADELPHIA. 315 



The group of frustules, moreover, is not always symmetrical, or arranged upon 

 the same plane. 



VI. Tryblionella? or Denticcla? n. sp. 



The form figured at No. 6 of the plate occurs sparingly in this deposit, and 

 more abundantly in the Wolfboro',but not in sufficient quantity to enable me 

 satisfactorily to determine its true generic position. It seems to be solitary. 

 Providing it be not a Denticulit, its analogies to Tryblionella are stronger than 

 to any other genus. The valvular surface is transversely and continuously 

 striate, with no indication of a central line. The costae are marginal. 



This diatom is very minute. 



Leaving a further consideration of this curious species, to which I hope to 

 be able to return on a future occasion, I shall briefly conclude this some- 

 what protracted notice, by calling attention to one or two points in connec- 

 tion with the mode of growth and extreme variation of a few of the known 

 species in the Saco mud. 



(1.) Eunotia hemicyclus I have sometimes found growing like Synedra luna- 

 ris : several frustules attached to a fixed point. E. hemicyclus is not abun- 

 dant in the gathering, and varies somewhat in the amount of its curve. The 

 extremities are usually bevelled off into a subacute conical point. (2.) Su- 

 rirella intermedia, in its smaller sporangial or embryonic form, commonly 

 occurs in groups of from three to four parallel frustules. I have not found 

 it in bundles like nomeocladia or Colletonema, but the position of the frustules 

 is somewhat suggestive of those thalloid growths. Navicula rhomboides is 

 similarly arranged. It presents a remarkable range of variation, as does N. 

 firma. (3.) Eunotia incisa is very variable in outline and striatiou. It shows 

 a tendency to unequal development at the extremities. These varieties or 

 anomalies I propose figuring in a paper on the Diatomaceai of the River 

 Delaware blue clay, to be shortly presented to the notice of the Academy, and 

 which was originally intended to be combined with the present communica- 

 tion. 



I have, in noticing these species, purposely placed them in the order in 

 which they occur, beginning with that most remote from the Synedroid type, 

 and ending with those which approximate most nearly to Synedra. What- 

 ever opinion may be entertained concerning the theory of the geological po- 

 sition of these forms, will not, I think, materially invalidate the following 

 points, which are fairly deducible from the premises : 



(1.) That the species and varieties in this deposit are singularly like, like 

 those of the sub-peat. 



(2.) That there is a notable absence of surface genera Nitzschia, Amphipleu- 

 rd, Tryblionella, Cymatopleura, Fragillaria, Odontidium, &c, &c, all of which 

 occur in adjacent localities. 



(3.) That certain exceptional forms are present, which appear to represent 

 types intermediate between the fossil Surirella and Eunotia, and the more 

 modern genus, Synedra (and Nitzsehiaf), Surirella Baileyi, S. intermedia, S. an- 

 ceps, S. delicatissima, illustrating the synthesis in the case of the former, 

 and Actinella in the latter, (Eunotia.) 



(4.) That these forms are exceedingly rare, and seem to be confined to locali- 

 ties having peculiar conditions of soil, which, in all likelihood, depend on 

 the mineral constitution of the water percolating through it. 



(5.) That there is an unusual tendency to variation in nearly all the spe- 

 cies, and a strong disposition shown by some of them to become attenuate 

 and elongated, and also to assume a marginal punctate arrangement, sugges- 

 tive of a Nitzschoid bias.* 



I may add that this same tendency to extreme variation affects the Desmidite which 

 abound in this locality. This is particularly the case with Triploceras verticiUatum , (Bailey), 

 which beautiful and showy Desmid presents every variety of outline and proportion reeoncileabl-i 

 with specific identity. The prevailing variety lilfifers from that fixed by Bailey, in the more un- 

 guieulate character of the terminal horns, and the inclination of the knot-like prominences. 



1863.] 25 



