Naviculacea.'] iNFrsoRiAL animalcules. 391 



nema, and Licmophora, form a group (the Stylhria of Agardh) dis- 

 tinguished by the ti-iangular form of the frustules, which have their 

 smaller ends directed towards a common centre. The frustules in. 

 this group have a central and two lateral portions, as in Diatoma and 

 Fragihria ; in which genera cuneate frustules are also occasionally 

 met with. But in Fragilaria or Biatoma, when two or more cuneate 

 frustules are united, the alternate frustules have their ends in oppo- 

 site direction, and hence thcii' filaments are linear ; whilst they are 

 attached, if at all, only by their basal frastule. In. this group, on 

 the contrary, as the smaller ends are in the same direction, they 

 point to a common centre, and, when stipitate, each frustule is 

 attached to the stipes. 



" Besides the two puncta at the broader end, two others, at the 

 smaller end, more obsciu'e, are generally present. The lateral surfaces 

 are attenuated at the base, which usually differs somewhat from the 

 upper end ; but in the group to which Fragilaria belongs, they are 

 similar at both ends, even when the frustules are cuneate." 



Mebidon circidare (Agardh.) — Frustules veiy minute, plane ; on 

 primary side (dorsum or face) without vittse ; on secondary side 

 (laterally) obovate -lanceolate, with distant, sti'ongly-markcd strise. 

 They are partly hyaline, partly of a yellowish-green colour. The 

 nearly circular bands occur in a mucous green stratum on mud, 

 stones, dead leaves, &c. ; in the spring, in mai-shes, stagnant waters, 

 and rivulets. 



This nearly = M. mrnale (Ehr.) — It has two conspicuous puncta 

 at the upper end, and from five to twelve along each lateral margin. 

 M. vernale. — Striated, wedge-shaped, truncate and dentate at the 

 interior (-wT-der) extremity, with two conspicuous puncta. Along 

 each lateral margin are from five to twelve vittae (Kiitz), producing a 

 beaded appearance. Fig. 177, is a spiral or nearly cii'cular band ; 

 fig. 178, a band, with some segments separating, seen on their sides. 

 Length of segment or frustule 1-11 50th to 1 -240th. 



M. (?) panduriforme. — Sinuous, wedge-shaped, of the form 

 of a violin. Length 1 -430th. Kutzing says its form is that of 

 Gomphonema acuminatum. 

 Meeidok Zincheni. — Primary side with vittae ; secondary side. 



