ANIMAL NATURE OF DIATOME.E. 411 



inclined to each other, by the different extension of the 

 two primary surfaces, the associated series must be formed 

 circularly, as it is effected in a circular or at least a 

 curved manner in the plane of the associated series, when- 

 ever the primary surfaces are cuneate, and the con- 

 vergence of the lateral surfaces is in the direction of one 

 extremity (Meridian, Odontidium, Diatoma.} 



27. ENCYONEMA. Cymbetta longitudinaliter seriate 

 tubo gelineo simplici tenerrimo molli inclusce. 



The gelatinous tube within which the Cymbellae 

 referred to this genus are included (E. paradoxum, E. 

 prostratum,} might perhaps be compared to the stipes of 

 Cocconema, and thus serve to explain its origin. We 

 must, in that case, suppose the stipes to represent the 

 gelatinous sac within which the Cymbellse is developed. 



In all the family of Cymbelleae (Cymbella, Cocconema, 

 Syncyclia, Encyonema,} we may repeat what has been 

 said before of the genus Cymbella, since the distinction 

 of genera is based upon characters merely accessory. 

 We may here refer to what Kiitzing says of the internal 

 substance. This is disposed in two lamina extended 

 upon the lateral surfaces, which present a median notch 

 (emarginaturd) corresponding to the convex side, and 

 are collected together into a very fine transverse mem- 

 brane. 



28. SPHENELLA. Individua solitaria, cuneata, libera, 

 nee affixa, nee stipitata, involuta. 



The Sphenellfs only differ from Naviculce in their 

 cuneate form, perfectly similar to that of Meridian, by 

 which, too, the associations (S. angustatd) become flabel- 

 liform and quasi circular ; but they differ from Meridian 

 by the central perforation of both secondary surfaces, 

 and by the interruption of the transverse striae of the 

 same surfaces, (8. ylacialis, S. vulgaris.} Hence there 

 remains a greater similitude to the Navicults, and the 

 distinctive characters are so slight, that the generic 



