172 
Mr. G. H. K. Thwaites on the Diatoinacese. 
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 Palmellea, 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 XL and XII. 
Plate XI. 
A. Meloscira varians, Ag. 1. Filaments with sporangia. 2. Filament 
consisting entirely of sporangial frustules. (Magnified 220 linear.) 
B. Aulacoseira crenulala. 1. Filament. 2. Filaments with sporangia. 
3. Sporangial frustules. (Magnified 220 linear.) 
C. Meloscira Borreri, Grev. Filament consisting partly of ordinary and 
partly of sporangial frustules. (Magnified 220 linear.) 
D. Cyclotella'l KiUzingiana. 1, 2. Frustules. 3. Frustules becoming con- 
verted into sporangia. 4. Sporangia. 5. Sporangial frustules. 
(220 linear.) 
Plate XII. 
E. Orthoseira Dickieii. 1, 1. Filaments. 2. Filament deprived of endo- 
chrome. 3. Filament 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. /3.). 
J. Schizonema neglectum. 1. Filaments (nat. size). 2. Filament (magnified 
220 linear). 3. Frustule. 4. Frustule deprived of its endochrome. 
Yi. Dickieia Danseii. 1. Portion of frond (magnified 35 linear). 2. Part of 
same (magnified 220 linear). 3. Frustule. 4. Frustule without 
endochrome. 
