31i DESCRIPTION OF {^PolygasincA- 



attenuated at eacli end, apices somewliat acute ; primary side larger, 

 linear- oblong, and ends rounded, ti'uncate ; transverse strias very 

 slender, seventeen in 1-1 200th. Length l-840tli to 1 -600th. AtThun. 



Cymbella Leptoceros = Cocconema Leptoceros (Ehi-.) — Minute ; 

 acute at the ends ; inflated at the centre ; primary side large, oblong- 

 elliptic, rounded at the ends; striie seventeen in 1-1 200th. Falaise and 

 America. 



C. affinu = Cocconema Fmidium (Ehr.) — Minute, somewhat obtuse 

 at each end ; dorsum more prominent, with large terminal apertures, 

 primary side larger, oblong-elliptic ; transverse stria3 distinct ; nine- 

 teen in 1-1 200th. Alive, Falaise, Schleswig. Fossil, America. 



C. ventricosa. — Minute, rather obtuse at each end, dorsum promi- 

 nent, rounded; venter plane, terminal apertures distinct, large, 

 hyaline; primaiy side oblong, truncate; striae indistinct. Length 

 1-lOOOth. Fresh water. 



C. excisa. — Eather small, dorsum prominent, convex ; venter with 

 a central notch ; apices produced, slightly recurved, obtuse ; trans- 

 verse striae sixteen in l-1200th. Length l-840th. On Oscillatoria, 

 Trieste. 



C. (?) Peclicidiis. — Very smoo^^. small, lunate; dorsum convex; 

 venter rather concave; ends sc*liewhat acute. The other aspe-ct 

 elliptic truncate. Common. Parasitic on Conferva. 



C. Q)flexella. — Small, smooth, secondary side elliptic, subsigmoid, 

 apices rather produced,but obtuse ; on the other aspect curved, convex ; 

 venter concave, notched (excised), ends truncate. Length 1 -650th. 

 At Thun, Switzerland. This species probably corresponds to Navi- 

 cula Semen, (Elir.) 



Genus DENTicELLA(Ehr.) — Generic characters unknown. 



This genus is not recognised by Kiitzing, who retains its species 

 mostly in Odontella, with which they were originally classed by 

 Agardh. Its affinities are with Idhmia, Biddulplna, and Zygoceros. 



D. aiirita. 

 D. fragilaria. 



D. gracilis. — Finely striated transversely ; width greater than the 

 length ; laterally constricted nearly to the central band ; chain 

 1-1 152nd in width. The length of D. aurita exceeds the breadth, 

 but both are compressed, and tridentate on the sides, with a dorsal 



