26 DIATOMACE.^R OF NORTH AMERICA. 



Frustules (inconspicuously) alate ; valves elliptical 

 or linear; not cuneate; parallel (dimidiate, mar- 

 ginal, or wavy) striate ; rarely costate ; median 

 line, if present, simple, and not beaded or con- 

 spicuously marked ; valves sometimes bent along 

 the longer axis (apparently ) ................ Tryblionella LTII. 



v. Frustules not as above ........................................ <5 



[ Frustules alate (sometimes inconspicuously); valves 

 cuneate, reniform, oval, or subcircular ; rarely 

 linear, frequently twisted ; with simple median 

 line, or more or less linear blank space ; center 

 j sometimes blank, or finely dotted ; margins or sub- 

 margins strongly marked (somewhat radiate); cos- 

 tate or plicate (canaliculate, W. N. ). ...... SURIRELLA LTV. 



All others (usually, finely striate >, linear, constricted, 

 inflated, sigmoid. beaked, fusiform or carinate ; 

 free, adherent, or in tubes ...................... NITZSCHIA LV. 



TRIBE III. CRYPTO RAPHIDIE.E. 

 FAMILY IX. CH^TOCERE.E. 



[ Frustules annulate ; cohering ; elongate ; ends alike, 

 calyptriform ; tipped with a spine or mucro ; often 

 imperfectly silicious ........................ RHIZOSOLENTA LVI. 



I Frustules not as above ........................................ 1 



( Frustules with horns (elongated processes ; not 

 1. spines simply); valves frequently unlike .............. 2 



(. Frustules with spines or bristles; or awns; or smooth. 3 

 c Frustules compressed, the sutural portion narrow ; 

 horns frequently branching or bifurcate ; some- 

 times niucronate ; valves and horns sometimes 

 with short scattered spines ; horns sometimes ob- 

 tuse, short (mammae "> .......................... DICLADIA LVII. 



Frustules elongated ; horns niucronate ; valves dis- 

 similar; generally one horn (or process) or one 

 valve, two on the other .................... Syringydium LVIII. 



( Spines 011 one valve only ; frequently long, and 

 3. -; sometimes branching ..................... SYNDENDRIUM LIX. 



' Frustules not as above ........................... ............. 4 



Valves angular ; spine central ; sutural portion more 



or less turgid (imperfectly silicious); valves with 



4. -j a radiate series of dots; frust ales connected in a 



distant series ........................................ DITYLUM LX. 



Valves not as above ............................................ f> 



