1 8 The American Geologist. July, i898 
Tetradium into four parts. He writes: "I know of only one 
fossil resembling Chcetctcs^ in which the tubes are multiplied 
by division, this is the genus Tetradium, whose tubes regular- 
ly divide into four parts." 
Later Nicholson* had repeatedly occasion to study the 
fossil and to discuss its affinities. 
In his excellent manual the characters of Tctradiwn are 
especiall}' clearly set forth as follows: 
"This small family includes only the single genus Tetradmm, which 
so far, has only been detected in the Ordovician rocks of North America. 
In this genus the corallum is massive, and is composed of long prismatic 
and closely contiguous corallites, which are of small size and have im- 
perforate walls. The tubes are furnished with longitudinal inflections 
or plications, generally four in number in each tube, which do not reach 
the centre of the visceral chamber, and which give a characteristic cruci- 
form or petaloid aspect to cross-sections of the corallites. These longi- 
tudinal plications are apparently to be regarded as of the nature of septa 
or pseudosepta. 7>/;77(^//«/« appears to be related to Halysites, but its 
true affinities and zoological position are uncertain." 
The following points in this description are of especial 
importance to us: Nicholson expressh' states that the plica- 
tions do not reach the centre and therefore are not to be 
compared to the longitudinal partitions projecting into the 
visceral chambers of the corallites of Chcetctes, which indi- 
cate the incompleted fission of the tubes, but that they are 
to be regarded as septa or pseudosepta. 
Further, Nicholson had observed beforef that the coral- 
lites are "in close contact, but not amalgamated by their 
walls" and that, hence, a double wall is observable. 
These observations induced him to consider Tetradium as 
not so closel}' related to C/icBtetes as its habitus would indi- 
*(i) H. A. Nicholson and Rob. Etheridge; On the genus Tetradhim, 
Dana, and on a British species of the same, in Ann. and Mag., 4th series, 
vol. XX, 1877. 
(2) H. A. Nicholson and Rob. Etheridge; A monograph of the Silur- 
ian fossils of the Girvan district in Ayi shire, fascicle i, 1878, p. 29. 
(3) Nicholson; On the structure and afifinities of the tabulate corals 
of the Palaeozoic period, London, 1879. 
(4) All. Nicholson and Rich. Lyddekker: Manual of Palaeontology, 
1889, p. 340. 
tCompare his papers on the Silurian fossils of the Girvan district 
and on the structure and affinities of the tabulate corals, etc. 
