24 DIATOM A CE.T: OF XORTH AMERICA. 



Valves finely striate. punctate, or more or less hya- 

 line ; never costate ; median line wanting 1 or ob- 

 scure; frustules narrow in f. v., sometimes inflated 

 14. J ( nndulato-arcuate) or constricted ; margins smooth, 

 cohering, forming a straight, sometimes zigzag, 

 filament ....................................... FRAGILARIA xxxn. 



Valves and frustules not as above ......................... 15 



f Frustules cuneate ; valves transversely costate or 

 distinctly granular striate, with a median line ..... 



PODOCYSTIS xxx rv. 

 I Frustules not as above ........................................ 17 



f Frustules compound ; valves costate or ribbed ; ends 

 of costae prominent, sub-marginal and capitate in 

 f. v. : frustules not cuneate .............. DENTICULA xxx \ . 



All others ; valves costate ; linear, or cuneate in 

 f. v.; cohering; filament zigzag, curved, or straight 



DIATOM A xxxvi. 



FAMILY VII. TABELLARIEJ-K. 



( Frustules linear, or cuneate, and in f. v., with inonil- 

 iforni vittae (ends of costae); valves divided into 

 chambers by transverse ribs (scalariform); outer 

 valve finely pervious striate, without median line. 



CLIMAC'OSPHENIA XXXVII. 



Frustules not as above ....................................... ] 



i. Frustules curved in f. v. ; valves costate ; dissimilar ; 



1. \ septae rudimentary ........................... Entopyla xxxvin. 



(. Frustules not as above ............................ 



f Frustules in f. v. with straight vittae, which are 

 usually alternate ; cohering, forming a zigzag fila- 



2. -j meut ; valves transversely striate, and inflated at 



center and ends ............................ T ABELLARIA xxxix. 



Frustules not as above ........ .' .................. 



c Frustules with straight vittae, in pairs, and inter - 



3. - rupted at the ends and center in f. v.. ..Diatomella XL. 

 (. Frustules not as above ............ ....... ...... 4 



I Frustules ith vittae in pairs, which are straight or 

 undulate in f. v. ; not interrupted or enlarged at 



4. -{ the ends: cohering, forming a zigzag filament ...... 



GR A:\nrvTOPHORA XLI. 

 [ Frustules not as above .......................................... "> 



f Frustules filamentous; vittae in f. v. somewhat en- 

 r larged at the ends (clavate); valves pervious cos- 



late : costae few. showing in f. v ...... G<>ii>i>li<>!/r<t>itm \i.n. 



Frustules not asahovr 



