154 MOLLUSCA. 
SHELL small, thin, crepiduloid, obliquely ovate, elevated, the vertex 
acute, projecting a little beyond the outline of the shell, its point in- 
curved and resting on the shell a little obliquely at the left, but not 
quite attaining the margin of the aperture; surface feebly striated by 
a few irregular lines of growth, and covered by a dusky-olive epider- 
mis. Aperture oval, the margin posteriorly expanded ; within oliva- 
ceous; diaphragm white, concave, at about one-third the depth of the 
cavity; the free edge acute, and its terminal spur contorted, so as to 
present its broad surface laterally. 
Length three-tenths of an inch; breadth one-fifth of an inch ; height 
one-eighth of an inch. 
Found in fresh-water streams near Wangarara, and at the falls of 
Waitanga, New Zealand. 
For a long time this curious little shell was regarded as a miniature 
form of some Navicella, though it was somewhat unexpected that this 
genus should be found in New Zealand. A closer inspection revealed 
its distinctive characters. In form it closely resembles Szphonaria 
Lessoni, especially the small, oblique varieties. 
Figures 176, 176 a, 1766, three views of the shell, enlarged; 176, 
the septum with its spiculum and canal, enlarged; 176 d, the shell, 
natural size. 
Genus NAVICELLA. 
The Navicell# were observed to prefer quiet little pools formed by 
eddies, adhering to the stones at some distance below the surface. 
They are nearly sedentary, and appear to shift their position little 
oftener than the Patellw; while the Neritine delight in the rapid and 
turbulent portions of the stream, and occasionally emerge altogether 
from the water, for a considerable time. 
Very numerous specimens of this genus were collected, though it 
may be doubted if a single well-characterized new species was ob- 
tained. A careful study of series of specimens taken from the same 
pool, has tended to show how very variable the characters of these 
