Devclopme7it of Tctradunn CcUidosiim Hall. — RuedcmaJin. 19 
cate, but to place it rather near the HalysitidcB, a view which 
had also been suggested by Safford.* 
Nicholson discusses the position of Tctradhim among the 
Tabulata more fully in his paper on the genus Tctradiiim, 
Dana, etc., where he writes: 
"Amongst the other groups of corals which have been generally referred 
to the miscellaneous division of the "Tabulata," it is difficult to find one to 
which Tetradiiim could be referred with entire propriety. - - - With 
the Halysitidce the genus has some d'ecided affinities, which are increased 
by Prof. Safford's observation that the corallum sometimes resembles 
that of Halysites or of Syringopora in form. We have not, however, 
noticed this mode of growth in any of the specimens which hive been 
examined by us, and the corallum in general is quite similar in form to 
that of the massive species of Favosites or ChcEtetes. Under anv circum- 
stances, should the genus be ultimately referred to this family, it will be 
in the immediate neighborhood of Halysites itself that it must find its 
place. - - - The only other group that needs consideration in this con- 
nection is that of the Chcetetidce. In general form and appearance there 
is the closest possible resemblance between the present genus and some 
of the massive forms of C/ia'tetes or Monticulipora\ but the peculiar sep- 
ta of the former are quite sufficient to distinguish them. Upon the 
whole, therefore, it is perhaps safest to regard Tet radium as an ally of 
Halysites, with some affinities to Chcstetes, and thus forming a connect- 
ing link between the families of the Halysitidce and Cka-tetido'." 
Nicholson's view became the prevailing one, for Miller 
and Zittel describe Tetradium as having four septa, which do 
not reach the centre of the visceral cavity and Zittelf places 
the genus in the appendix, containing "genera from the 
group Zoantharia Tabulata, E. H., of entirely doubtful sys- 
tematic position." 
In later )ears, however, Prof. M. Neuma^r^ advanced a 
different view, viz. that Tetradium and Ch(Etctcs are closely 
related, that the composition of the walls of Tetradium of 
two lamellae, which constitutes the principal difference be- 
tween the two genera, has not yet been conclusively proved, 
*Safford (op. cit., p. 236) writes "The tubes are most frequently united 
throughout laterally, forming massive coralla resembling more' or less 
those of Favosites and Chadctcs; sometimes, however, they are united 
in single intersecting series, as in Halysites catctiulatus, Liiin., not infre- 
(juently too the tubes are isolated, or only united at irregular intervals, 
thus forming loose, fasciculated coralla resembling certain forms of 
Syringopora^ 
tHandbuch der Pakeontologie, PaUtontologie, Bd. i, p, 6ig. 
JDie Stiimme des Thierrciches, Wirbellose Thiere, vol. i, i88g, p. 317. 
