NAVICULA CLEPSYDRA. 68 
Prit. Inf. p. 914, pl. vii. fig.77; Rabenh. Europ. Diat. p. 251; 
Heib. Cons, p. 84. n. 6. 
Plate X. figs. la, 1b, & le. 
Frustule large, elongated, on M. V. oblong, constricted in 
the middle, angles much rounded, extremities truncate. 
V. linear-lanceolate or elliptical-lanceolate, with ob- 
tuse or subacute extremities ; striz oblique, distinct, 
and granular, 20 in 001", reaching to the median 
line, much shortened (so as to form a narrow, linear, 
marginal hand, and to leave a cuneiform blank space) 
opposite the central nodule. Dry valve tawny-co- 
loured. ., 
This species, originally placed by Ehrenberg in his genus 
Stauroptera, and afterwards by Kiitzing and subsequent 
observers in the genus Stauroneis, is a genuine Navicula, 
and so closely allied to N. Clepsydra, Donk., that to place 
these two species in different genera would be a violation 
of correct classification ; the proper position for both forms 
is in the present subsection. Prof. Walker Arnott (Quart. 
Mier. Journ. vol. vii. p. 176) correctly pointed out that 
the striation in both species is different from what is 
usually observed in the genus Stauroneis. N. aspera is 
very variable in size, the small variety figured being 
evidently the Stawroptera Achnanthes of Ehrenberg. 
Hab. Marine: frequent in rocky pools, and generally 
distributed. 
Navicula Clepsydra, Donkin. 
Navicula Clepsydra, Donk. Quart. Micr. Journ. n. s. vol. i. p. 8, pl. i. 
fig. 3; Rabenh. Europ. Diat. p. 181. 
Plate X. figs. 2a & 26. 
Frustule large, elongated, on M. V. broad, quadrangular, 
