388 MR. G. 8. WEST ON 
this resulting in à conspicuous alteration in the form of the sinus 
(Pl. 10. figs. 11, 14, 17). Mauy stages were observed in the 
division of the cells, and it often happened that a second division 
of the cells commenced before the first was completed. This 
sometimes continued until several immature cells intervened 
between the original adult semicells (Pl. 10. figs. 14, 15). A 
precise parallel to this is seen in Staurastrum brachiatum, 
Ralfs (cfr. West & G. S. West in Journ. Roy. Mier. Soc. 1896, 
p. 159, t. 4. ff. 55-56). In some cases division had taken place 
in one of these undeveloped cells whieh had previously become 
free, and on the completion of this division the newly-formed 
semicells were typical, showing that if a characteristic feature of 
a species be absent from any individual, it may be produced at 
its maximum in the semicells of a sueceeding generation (Pl. 10. 
figs. 16, 17). 
In a gathering of an immense quantity of this species from 
Riccall Common, E. Yorks., the specimens were all of a rather 
large size (long. 10-12°5 p; lat. 8'6-10°5 u; lat. isthm. 376—465 u; 
crass. 5 pu), and the superior angles were in many cases emar- 
ginate (Pl. 10. figs. 18, 19). 
From Pilmoor, N. Yorkshire, two forms were seen: one small 
form with much rounded semicells (long. 6'6 u; lat. 66-74; 
lat. isthm. 3'7 p; crass. 3:9 u), which might be regarded as var. 
tritum, West & G. S. West in Trans. Linn. Soc., Dot. ser. 11. 
vol. v. 1895, p. 59, t. 9. f. 24 (PI. 10. fig. 20); and another very 
large form with somewhat rectangular semicells and frequently 
with emarginate superior angles (long. 10:9-11:7 p; lat. 10°1- 
10°9 u; lat. isthm. 4°6-5'6 p; crass. 5'4 a). This is the plant 
described by A. W. Bennett as Zuastrum crenulatum* in Journ. 
Roy. Micr. Soc. 1887, p. 17, t. 4. ff. 20-21. 
Great variation is met with in the vertical view of this species, 
some examples appearing quite elliptical, others having slightly 
tumid lateral margins and rounded poles, and yet others possess- 
ing a prominent protuberance in the middle on each side and a 
distinct, though smaller, one on each side near the poles (cfr. 
Pl. 10. figs. 11, 19, 20, 21, b). The extent to which these pro- 
tuberances are developed varies greatly even in specimens from 
the same locality, and this has induced me to remark upon a very 
interesting point. 
* Dennett's figure of the side view of this plant is incorrect. 
