380 MR. G. 8. WEST ON 
have not been able to determine definitely whether the striola- 
tions are the thickest or the thinnest parts of the cell-wall, but 
there is every indication that they are internal thickenings. In 
the majority of instances the edges of the striolations are not 
smooth, but exhibit various degrees of roughness, and in some 
specimens under a particular focus a reticulation is perceived 
to exist between the punctulations apparently connecting the 
striolations together (PI. 8. fig. 11). 
From these facts it is clearly evident that the three species sepa- 
rated by Turner, and based upon differences in the strength of 
striolations, are merely forms of one and the same plant. The 
striolations have been shown to vary considerably in number 
and strength in different individuals in the same gathering, even 
on different parts of the same individual, and upon these cha- 
racters Turner based his separation of Penium spirostriolatum and 
P. scandinavicum. In some examples the striolations frequently 
become discontinuous, forming series of dots (cfr. Pl. 8. figs. 3, 4, 
& 6); and as this character was utilized to separate P. Royanum, 
this species must also be regarded as a mere form of P. spiro- 
striolatum. 
9. CLOSTERIUM STRIOLATUM, Ehrenb.—The small amount of 
variation met with in specimens of this species is mainly found 
in the comparative breadth of the apices and the middle portion 
of the cell, and in the striolation of the eell-wall. The following 
table shows this variation in a series of specimens from three 
localities :— 
Breadth com- Maximum Minimum 
pared with breadth of | breadth of | Number 
| Specimens from length apex com- | apex com- | of strio- 
| (average of 25 pared with | pared with {lations in 
s ecimens) diameter of | diameter of | 20 p. 
| P i cell. cell. 
| um 
| Epping Forest, Essex.| 1:797 1:241 1:38:02 13 | 
Clapham, Yorks, ......| 1:865 1:245 | 1:251 | 15 | 
Frensham, Surrey...... 1:11:64 1: 207 1:275 14 | 
| 
The specimens from the Surrey locality were not only longer 
