NATURAL SCIENCES OP PHILADELPHIA. 



339 



Therefore, before leaving S. Baileyi, I desire to call attention to the Synedroid* 

 character of the smaller varieties, together with their apparent degradation 

 of generic character, as points which will be found intensified in the species 

 next to be described. 



II. Surirella intermedia, n. sp. Frustules free. Valve linear, strongly 

 sigmoid with attenuate rounded apices. F. V. straight or slightly sigmoid, 

 expanding at the subtruncate extremities. Alse usually distinct, twisted 

 near the ends of the valve, giving rise to aspathulate appearance. Canaliculi 

 numerous, inconspicuous, reaching the narrow central blank line. Strite 

 distinct, variable as to number and fineness, as in S. Baileyi. For the same 

 reason I have not estimated their average. Fig. 2, a, and b. 



Var. /?. With the same general outline, only much smaller. Alse and 

 canaliculi rudimental. Occurs in groups of from two to five; probably spo- 

 rangial. 



This remarkable species, more curious and anomalous in its character than 

 the preceding, is quite abundant in the pond. For a good while I was dis- 

 posed to regard it as an aberrant variety of Nitzschia or Amphipleura, but a 

 more careful study of the structure of the valve led me to refer it to Surirella. 



The points of resemblance which seem to me to place it in that genus are : 

 the presence of well-marked alze and of tolerably distinct canaliculi, structural 

 appendages never found in Synedra, Nitzschia or Amphiplcur a, to which genera 

 S. intermedia is allied by external configuration and curvilinear attenuate pro- 

 portions. Of a certain superficial resemblance to the punctulation of Nitz- 

 schia, visible in the small frustules of what may be regarded as the sporangial 

 brood of aS. intermedia, it is hardly necessary to speak, as a careful study of 

 the intermediate varieties of this diatom shows that these seeming puncta are, 

 in reality, only miniature alse. 



Before going further, it may be as well, for convenient reference, to give 

 the definitions of those genera most nearly allied to this species and its va- 

 rieties. In order that the relative importance of their respective points of 

 resemblance to the new species may be more readily apprehended, I have 

 italicized those special characters of each genus represented in 5. intermedia: 



Surirella. 



Synedra. 



Nitzschia. 



Amphipleura. 



(Says Prof. Smith,) is 

 distinguished by the fol- 

 lowing characters : 



" Frustulusfree, ovate 

 or elliptical. Valves with 

 a longitudinal central 

 line and margins pro- 

 duced into alx. Canali- 

 culi distinct, usually 

 parallel." He further 

 adds that, " It is well 

 distinguished from Try- 

 blionelta by the promi- 

 nence of its alze and the 

 usual cuneate form of 

 its frustules. With no 

 other form is it likely to 

 be confounded" 



" Frustules elongated 

 wand like, attached by 

 the lower end. Lateral 

 surfaces equal to or less 

 than the front view: 

 traversed by a smooth 

 line." 



(Last character omit- 

 ted by Prof. Smith.) 



" Frustules free, elon- 

 gated, compressed. 



"Valves linear, keeled 

 with one or two longi- 

 tudinal lines of puncta. 

 " Keel frequently ec- 

 centric." 



"Frustules free, li- 

 near, with longitudinal 

 ridges." (Vide Surirel- 

 la anceps. n. sp.) 



Generic characters ad- 

 mitted to be obscure by 

 Prof. Smith. 



It will be seen, by reference to the italicized portion of these definitions, 

 that although the characters belonging to these genera, represented in S. inter- 

 media are about equal in number, this is far from being the fact as regards 

 their structural significance, which in the case of those noted in Surirella is 

 much more considerable than in any of the others. 



* I employ the term "Synedroid' here and elsewhere in this notice, to designate these forma, 

 as, although the apparent punctulation in some of them is more suggestive of Nitzschia than of 

 Synedra, they never present any trace of keel. 



18G3.] 



