SEPTEMBER 25, 1913| 
NATURE 113 
country materially decreases the gap in our know- 
ledge of the succession of forms belonging to this 
genus, which had previously existed. 
Girvanella. 
This organism, which is now known to be widely 
distributed in the Palaeozoic and Mesozoic rocks of 
this country, was originally described in 1878 by 
Nicholson and Etheridge, jun., from the Ordovician 
rocks of the Girvan district. The genus was estab- 
lished to include certain small nodular structures 
composed of a felted mass of interlacing tubes, having 
a width of ro and 18 », the cells being typically simple, 
imperforate tubes without visible internal partitions. 
The geno-type, G. problematica, was, however, at that 
time referred to the Rhizopods and regarded as related 
to the arenaceous foraminifera (‘Silurian fossils in 
the Girvan district,’ 1878, p. 23). In 1888 Nicholson, 
in redescribing this genus in the Geological Magazine, 
compares Girvanella with the recent form Syringam- 
mira fragillissima of Brady. 
More recently Mr. Wethered has shown that an 
intimate association frequently exists between Gir- 
vanella tubes and oolitic structure, and he has 
described several new species of Girvanella, from the 
Palzozoic rocks and also from certain Jurassic lime- 
stones. 
The reference of Girvanella to the calcareous alge, 
though not yet supported by incontestable evidence, 
has been advocated by several writers in recent years. 
Even as long ago as 1887, Bornemann, in describing 
examples of Siphonema (Girvanella Nich.), which he 
had discovered in the Cambrian rocks of the Island of 
Sardinia, suggested that this organism might belong 
to the calcareous algz. 
In 1891 Rothpletz noticed that some of the speci- 
mens of Girvanella which he had examined were 
characterised by dichotomous branching of the tubes; 
on this account he removed the genus from the Rhizo- 
pods to the calcareous algz, placing it provisionally 
among the Codiacee. Three years later Dr. A. 
Brown, in summing up the evidence in favour of the 
inclusion of Solenopora among the nullipores, ex- 
pressed the opinion that Girvanella might ultimately 
come to be regarded as referable to the Siphonez 
Verticillatz. 
In 1898, however, this genus was still only doubt- 
fully placed with the calcareous alge, for Seward. in 
his work on fossil plants,’ remarks: ‘‘The nature of 
Girvanella, and still more its exact position in the 
organic world, is quite uncertain. ... We must be 
content for the present to leave its precise nature still 
sub judice, and, while regarding it as probably an 
alga, we may venture to consider it more fittingly 
discussed among the Schizophyta than elsewhere.” 
In 1908, however, Rothpletz, in discussing the rela- 
tionship of Spherocodium and Girvanella, reaffirms 
his opinion that the latter must be referred to the 
Codiacez.*® 
Mitcheldeania. 
This genus was first described by Mr. Edward 
Wethered from the lower Carboniferous beds of the 
Forest of Dean® under the name of Mitcheldeania 
Nicholsoni; it was referred by him to the Hydrac- 
tinidze, and considered to be allied to the Stromato- 
poroids. The figure accompanying this paper unfor- 
tunately fails to show any of the characters of the 
organism, but a better figure of the same species was 
subsequently published in the Proceedings of the 
Cotteswold Naturalists’ Field Club.’? 
7 Vol. i, p. 125. 
8 Rothpletz. *“Ueber Algen und Hydrozoen,” of. cit. 
9 “Geol. Mag.,” Dec. 3, 3, cccccxxxv, 1886. 
10 Vol. ix. p. 77, pl. v., 1886 
NO. 2291, VOL. 92] 
In 1888 Prof. H. A. Nicholson published in the 
Geological Magazine figures and descriptions of a 
new species of this genus (M. gregaria), and redefined 
the genus as having ‘‘the form of small, rounded, or 
oval calcareous masses made up of capillary tubes of an 
oval or circular shape, which radiate from a central 
point or points, and are intermixed with an interstitial 
tissue of very much more minute branching tubuli.” 
He compares the larger tubes to zooidal tubes, and 
states that they ‘‘communicate with one another by 
means of large, irregularly-placed foramina resembling 
“the mural pores” of the Favisitida, and they occa- 
sionally exhibit a few irregular transverse partitions 
or tubule.” 
With regard to the systematic position of this 
genus, Nicholson remarks: ‘In spite of the extreme 
minuteness of its tissues, the genus Mitcheldeania 
may, I think, be referred with tolerable certainty to 
the Coelenterata . . . its closest affinities seem to be 
with the hydrocorallines . . . on the other hand, all 
the known hydrocorallines possess zooidal tubes which 
are enormously larger than those of Mitcheldeania; 
and there are other morphological features in the 
latter genus which would preclude its being actually 
placed, with our present knowledge, in the group of 
the Hydrocoralline.”’ 
Since this description by Prof. Nicholson, no further 
account of this organism, so far as I am aware, has 
been published, and its reference to the Hydrozoa rests 
on Prof. Nicholson’s description. 
During the past few years I have collected a large 
amount of material from both of the type localities 
from which Mr. Wethered and Prof. Nicholson ob- 
tained their specimens, and an examination of this 
material has impressed me strongly with the re- 
semblance of Mitcheldeania to forms such as Soleno- 
pora and Girvanella, now usually classed among the 
calcareous alge. In the rocks in which it occurs 
Mitcheldeania appears as rounded and _lobulate 
nodules, breaking with porcellanous fracture and 
showing concentric structure on weathered surfaces, 
very similar to nodules of Solenopora; while under 
the microscope the branching character of the tubules 
and their comparatively minute size appear to separate 
them from the Monticuliporoids. Prof. Nicholson 
appears to rely on the presence of pores, which he 
thought he observed in the walls of both the larger 
and finer tubes, for the inclusion of this genus with 
the hydrocorallines, though he appeared to be doubt- 
ful about their occurrence in the interstitial tubuli. 
An examination of a large number of slides has failed 
to convince me of the presence of pores, even in the 
larger ‘‘zooidal tubes.’ The large “ oval or circular” 
apertures noticed by Nicholson appear to be either 
elbows in the undulating tubes cut across where these 
bend away from the plane of the section, or 
places where a branch is given off from a tube 
at an angle to the plane of the section. If this view 
be accepted, there appears to be no sufficient reason 
why Mitcheldeania should not be ranged with Soleno- 
pora and other similar forms, and included among the 
calcareous algae—a position which its mode of occur- 
rence and general structure has led me, for some 
time, to assign to this organism. 
In addition to the three chief forms described above 
from British rocks, a study of numerous thin sections 
from the Lower Carboniferous rocks of the north- 
west of England has revealed the presence of several 
distinct organisms, which will, I think, eventually be 
found to be referable to the calcareous alge. 
This meagre list appears to exhaust the genera 
known at the present time from the Lower Car- 
boniferous rocks of Britain, while the only additional 
genus so far recorded from the Mesozoic and Tertiary 
