NATURAL SCIENCES OF PHILADELPHIA. 



15 



Nitzschia ; subjective genera, Surirella, Himantidium, Odontidium, Tabellaria, 

 Eunolia, and resultant varieties. (PI. ii., figs. 1, 2, 12, 13). 



(5). An extreme variation, not special in its direction towards any type or 

 genus, extending to the valvular outline of many altogether distinct genera, 

 characterizes these metamorphic epochs ; manifesting itself in disposition to 

 assume an undulate, apiculate or crenulatc contour, or to lose these characters, 

 e. g. Eunotia incisa, Himantidium, Navicula serians, iV. firma ; or in irregular, 

 abnormal or unsymmetrical forms, due probably, to unequal or spasmodic 

 action of the metamorphic force, e. g. Actinella, Tetracyclic, (fig. 3, pi. ii.) 



(6). These so-formed varieties, where based upon changes of generic char- 

 acter, or where abnormal or unsymmetrical, are always short-lived; on the 

 other hand, varieties not involving generic character, often become fixed and 

 permanent, as Navicula firma, (var. iridis, amphigomphus, c.) N. serians, var. 

 /?., N. rhomboides. 



(7). That it is not improbable, that many species not reputed distinct, are 

 capable of conjugating with each other ; and that peculiarities of outline and 

 striation constitute but uncertain data for division of species. 



Table of Species. 

 (*) Indicates the presence of species at the locality. 



Localities where the " intermediate 

 type" species occur. 



Surirella Baileyi 



" intermedia 



" anceps 



" delicatissima. 

 Actinella punctata 



1865.] 



