80 Dr. Gregory,, on the Post-tertiary 
line. S. V. nearly rhombic, narrow ; but the angle in the 
middle is, as it were, rounded off, and not well marked as in 
N. angularis. Extremities very acute. Keel central, pro- 
minent, with the same distant markings as appear on the 
F. v., only here they are seen as puncta. Length about 
0-003". (296.) 
45. Nitzschia socialis,ia.. sp. This pretty species is remark- 
able from its occurring, in the prepared material, after boiling 
with acids, in groups of six, eight, ten, twelve, or more, with- 
out any apparent connection between them. One of these is 
represented in fig. 45. 
Form of the S. V. linear lanceolate, narrow; extremities 
acute. Keel central. Puncta closely set. Valve finely but 
very distinctly marked with transverse parallel striae ; striae 30 
to 36 in O'OOl". F. V. rectangular. Colour yellowish-brown. 
Length about 0'0032". The valves also occur detached ; but 
the groups are very frequent. This form may possibly be a 
Homseocladia, but it seems quite distinct from any of those 
figured by Professor Smith. (297.) 
46. Nitzschia insignis, n. sp. This fine species is not very rare 
in the deposit, though often fractured, and the F. V. is very sel- 
dom seen. The F. V. is shown in fig. 46, the S. V. in fig. 46"^. 
Form of the F. V. linear, rectangular, with obtusely rounded 
ends. Puncta distant. Lateral parts transversely striated. 
Strise conspicuous, especially on the S. V., where they traverse 
the whole valve. Length from 0*007" to 0 015". Puncta 
about 6 in 0-001". Strise about 30 in 0-001". Keel on the 
S. V. not quite central. S. V. slightly bent at the ends, opposite 
ways, in some individuals ; in others perfectly straight. 
This species, in length, and also in form, resembles the 
straighter forms of N. sigmoidea, from which the conspicuous 
striation at once separates it. From N. BrSbissonii it is dis- 
tinguished by the aspect of the F. V., which in N. insignis is 
much longer and narrower, and quite straight, besides having 
a long blank rectangular space in the middle. In N. BrSbis- 
sonii the F. V. is striated across the whole valve, except a 
narrow raphe, and the valve is sigmoid on this view. It 
cannot be confounded with N. scalaris, having much finer 
markings, and being a much slenderer form than that species. 
It evidently belongs, however, to the same group. It is 
further distinguished from N. scalaris by the absence of the 
rounded swellings out near the ends of the F. V. (298) . 
I have not yet found this, or either of the two other species 
of Nitzschia here described, in any recent gathering. They 
are probably all of marine origin. 
(AH three species occur in the recent gathering above named.) 
