312 DESCRIPTION OF '^JPolyrjasinca, 



disc l-480tli. Fossil in the Elbe deposits. Probably a segment of 

 Mehsim arenaria. 



Cyclotella (t) Kutztnffiana (Tbwaites.) — Frustules (cells, on primary 

 side sigmoid, flexnose ; on secondary side with radiating striae. The 

 frustules of this species are short, and have an apparent sigmoid 

 curvature, which is due to each of their striated, discifonn ends, 

 having a prominence on one side of its centi'e, and a dej)ression on 

 the other, and the opposite end of the frustule having a depression 

 and prominence corresponding to these. The sporangia are developed 

 much in the same way as in Melosira. This species is closely allied 

 to C. (?) minutula (Kiitz.), but differs in the curvatures apparent 

 in the ifrustulcs. Itoccui's in brackish water, near Bristol. (Ann. N. 

 H. 1848, p. 169.) 



Genus Cymatopletjea (Smith). — Yalves undulated, margins not 

 produced into alae. Frustules free, solitary, or, when undergoing 

 self- division, in pairs. 



It has the habit of SurireJla, but the undulated siirface of the 

 valves seems to indicate a peculiarity of structure sufiicient to con- 

 stitute a generic difference, and the absence of ala3 and costse implies 

 a further diversity in the internaL-haracter, which cannot be regarded 

 as unimportant, (Ann. Nat. HisiT^p. 12. 1851.) 



C. solea = Surirelh lihrilis (Ehr. P. 18, f. 9, a lateral view.) 



C. elJiptica = Surirelh elJiptica (Kiitz.) — Frustules on front view 

 oblong, linear ; on side view broadly elliptical, surface of the valves 

 with about four imdulations, obsciu-ely striated. Length l-350tli 

 to 1 •200th. Breadth about half the length. Fossil and alive in 

 slow streams or ponds with OsciUatoriece (P. 18, f. 7, 8.) It is veiy 

 variable in size, the fossil specimens being usually twice as large as 

 the recent frustules. Both extremities of the valves are somewhat 

 pointed. 



C. Hihernica. — Frustules on side view orbicular, with prominent, 

 somewhat pointed extremities; surface of valve with about three 

 undulations obscurely striated. Length l-370th to l-220th. Breadth 

 two-thirds the length. Eiver Bann, Ireland. 



Genus Cymbella (Kiitz.) — Individuals solitary or geminate, free 

 (neither adnato nor included), unequally curved ; one primary side, 



