Naviculacea.'] iNFrsoEiAl, ANiMALCtrLFfl. 379 



Melosira crenulata. — Similar generally to the last, of which it is 

 little more than a variety ; but the long diameter of its segments 

 is two to four times greater, and the margin clearly denti- 

 culate. Diameter 1- 1440th. England, North and South America. 

 (P. 14, f. 29.) 



M. Bindsrana. — Another variety ; more slender ; joints variable, 

 sometimes ventricose ; length four to eight times the greater ; 

 junction-margin minutely striated. Diameter l-6000th. to l-2400th. 

 Hamburg. 



M. Americana. — Size moderate ; all the segments united in a 

 cylindi-ical tube, separated only by septa; transversely striated on 

 the margin, and divided by a median sulcus. Diameter 1 -660th. 

 Tropical, America, 



M. decussata. — Slender, joints cylindrical, the two diameters 

 nearly equal ; remarkable in being spirally decussated by numerous 

 very finely dotted lines. Shores of the Elbe. 



Sub-Genus AxTLACOSEiRA (Thwaites), (Ann. Nat. Hist. 1848.) — Cells 

 cylindrical, bisulcate, extremities more or less orbicular, concatenated 

 in filaments. 



Aulacoseira differs from Melosim by the deficiency of the centi'al 

 line, the place of future fissiparolis division. Each of its frustules 

 too, have two somewhat distinct sulci or fossulae (furrows, E-alfs) 

 passing round it. The absence of the central line separates Aula- 

 coseira also from Orthoseira, from which it is further distinguished 

 by the convex ends of its cells or frustules. 



Its typical species is the Melosira crenulata (Kiitz.) the M. auri- 

 chalcea (Ralfs); A. crenulata (Thwaites.) Characters those of the 

 genus. Sporangium spherical, with its axis of elongation at right 

 angles to that of the frustide fi-om which it originated. Around 

 the young Sporangium a considerable quantity of mucus is developed, 

 by which the empty half-frustules are for some time held attached. 

 (P. 24, fig. 33,) represents filaments of Aulacoseira crenulata with 

 Sporangia, 



Sub-genus Oethoseiea (Thwaites.) — Cells exactly cylindrical, 

 with a central line, connected in cylindrical filaments; internal 

 cavities spherical or sub-spherical. 



cc 2 



