392 DESCEIPTION OF [Poli/ffasfrira. 



obovate-lanceolate ; the filament formed is curved, or nearly 

 straight. Length l-420th (P. 16, f. 21.) 



Meridon (?) Ovatum (Agardh.) — "Frustules ovate, combining 

 to form a cellular band." Kiitzing mentions this as a doubtful 

 species, described by Agardh. 



M. comtrichim (Ualfs, Ann. Nat. Hist., 1843, p. 458.) — Lateral 

 surface constricted below the apex, ti'ansversely striated, the ends of 

 the striae forming puncta along the margins of the front view. 

 Frustules united together in a chain so as nearly to form a circle, 

 but are not arranged on a level plane, as M. vernale, standing nearly 

 erect, after the manner of the staves of a tube. They agree in size 

 and fonn, and in the puncta with M. vernale, but differ most remark- 

 ably in the constriction below the apex. Found by Mr. Jenncr at 

 Tunbridge Wells. 



Genus Monogeamma (Ehr.) — Lorica -s^dth transverse pinnules 

 (stiite), and a central transverse linear space, but only traversing one 

 half the -width (a semi-crucial umbilicus) ; with tln-ee ventral and 

 two dorsal apertures. 



2I()no(jramma, therefore, equals Stauroptera, but with a semi- 

 crucial umbilicus ; it has also a resemblance to solitary Achnanthes, 

 with terminal apertures. y 



Species unknown. 



Genus Nayicula (Bory St. Vincent.) — The little skip Animalcules 

 derived their generic name from the resemblance in form of the 

 many species to a weaver's shuttle. According to Ehrenberg, it 

 comprehends those members of the family Bacillaria which are 

 unattached, and have a simple bivalved, or multivalved lorica. 

 They occur single, or in pairs, but are never united in the form of a 

 chain. The lorica of Navicula is a closed, mostly-four-sided, hard, 

 and glass-like little bivalve case {testula hivalvis), which, in drying, 

 often separates; when lightly pressed, it breaks or divides either 

 into two or four longitudinal parts ; sometimes the angles are pro- 

 vided with a short rib, distinctly ftirrowed, the lorica then separates 

 into four equal parts ; but in some cases the two rows of ribs 

 arc not visible, the two halves of the lorica being obliquely fun-owed ; 

 it then separates into two parts. By heating the body upon platiua- 



