172 Mr. G. H. K. Thwaites on the Diatomacese. 



with a linear space on either side of the central mark striated. 

 This beautiful new species is extremely interesting as illustrating 

 the real structure of the genus Dickieia. Each frustule developes 

 around itself a definite amount of gelatine, so that at each repe- 

 tition of fissiparous division additions are made to the amount of 

 gelatine of the frond by the new frustules which are then pro- 

 duced. In the present species these additions are in the form of 

 mammillae, and a good deal resemble the mucous prolongations of 

 some of the Palmellece, a frustule being situated towards the extre- 

 mity of each. A mammillose and somewhat areolate appearance 

 is thus given to the indefinite frond, whereas in Dickieia ulvoides 

 the newly developed additions to the gelatine cohere to form a 

 compact even membrane. 



I have great pleasure in naming this species in compliment to 

 its discoverer George Dansey, Esq. of Devonport, who finds it in 

 small quantity upon rocks on the tidal shore of the river Tamar. 



EXPLANATION OF PLATES XI. and XII. 

 Plate XI. 



A. Meloseira varians, Ag. 1. Filaments with sporangia. 2. Filament 



consisting entirely of spovangial frustules. (Magnified 220 linear.) 



B. Aulacoseira crenulata. 1. Filament. 2. Filaments with sporangia. 



3. Sporangial frustules. (Magnified 220 linear.) 



C. Meloseira Borrerl, Grev. Filament consisting partly of ordinary and 



partly of sporangial frustules. (Magnified 220 linear.) 



D. Cyclotellat Kiitzingiana. I, 2. Frustules. 3. Frustules becoming con- 



verted into sporangia. 4. Sporangia. 5. Sporangial frustules. 

 (220 linear.) 



Plate XII. 



E. Oi'thoseira Dickieii. 1,1. Filaments. 2. Filament deprived of endo- 



chrome. 3. Filam.ent the terminal cells of which are becoming 

 converted into sporangia. 4. Immature sporangia. 5. Sporan- 

 gium. 6. Sporangium become fissiparously divided. 7. Sporan- 

 gial frustules becoming developed from one of the halves of a 

 sporangium. (Magnified 220 linear.) 



F. Schizonema eximium. 1. Filaments (nat. size). 2. Portion of filament 



(magnified 220 linear). 3. Frustule. 4. Frustule deprived of its 

 endochrome. 



G. Schizonema subcohcerens. 1. Portion of plant (nat. size). 2. Part of 



same (magnified 20 linear). 3. Part of filament (magnified 220 

 linear). 4. Frustule. 5. Frustule without endochrome. 6. Spo- 

 rangia. 7. Sporangial frustule. 



H. Schizonema vulgar e. 1. Filaments of var. a. (nat. size). 2. Filament 

 (magnified 220 linear). 3. Frustule. 4. Frustule without endo- 

 chrome. 5. The same (of var. j3.). 



J. Schizonema neglectum. 1. Filaments (nat. size). 2. Filament (magnified 

 220 linear). 3. Frustule. 4. Frustule deprived of its endochrome. 



K. Dickieia Danseii. 1, Portion of frond (magnified 35 linear). 2. Part of 

 same (magnified 220 linear). 3. Frustule. 4. Frustule without 

 endochrome. 



