57 
Tryplasma. Only one fundamental difference exists between Pholidophyllum 
tubulatum and the species I have referred to Tryplasma, both Australian and 
exotic. In no description of any of the latter have I noticed any reference 
to the exothecal structures described by Lindstrom adhering to the outer 
walls, or at the edge of the calice in P. tubulatum, and, similarly, there is no 
trace of these in any of the Australian forms. These scales were regarded 
by Lindstrom as homologous with the opercula of llhizophyllum and its allies. 
If, therefore, the presence of such scales is essential to the stability of 
Pholidophyllum, and further, if Tryplasma, Lonsdale, is the equivalent of 
Lindstrom’s genus, then I am fain to confess that our Australian corals 
cannot be so referred. The whole matter hinges on the question, are these 
exothecal scales homologous with the opercula of llhizophyllum, Calceola, 
&c. ? In the well-considered words of a reviewer of Liudstrom’s paper — 
“The Operculate Corals of the Palaeozoic Formation” — “the walls were 
covered with vertical rows of regularly disposed, minute, overlapping 
calcareous scales or plates, oval or pear-shaped in outline, minutely striated 
on the outer surface, and with a small process on the inner surface as if in 
connection with a muscular attachment. The plates are in paired rows, each 
pair covering over the slight vertical furrow between two rugae or costae. 
Judging from their sections, one side of each scale seems to have been slightly 
embedded in the w r all of the coral, but the attachment must have been slight, 
for examples in which these scales are preserved in situ are very rare.” The 
reviewer added, “ which in a certain sense are homologous with the opercula 
of the genera above mentioned.” 1 
These lids, or “ opercula” of the Palaeozoic Operculate Corals, were 
also likened by Lindstrom to the opercular valves of the living genera 
Primnoa and Paramuricea . 2 * lie remarked : — “ I think there exists some sort 
of homology to these ectodermic scales of the Primnoae amongst the Rugosa 
in Cyathophyllum ( Pholidophyllum , n.g.) Loveni, Edw. and II ., 5 specimens 
of this very common and widely distributed fossil show, when in a good state 
of preservation, a thick covering of small (| mm.), very thin scales, tightly 
clustered together in longitudinal rows along the costae 
This position of the scales on the theca or epi theca of the coral gives them 
an exothecal character.” 4 
1 G.J.H.— Geol. Mag., 18S3, X (2), p. 133. 
2 Lindstrom — Geol. Mag., 1871, VIII, p. 122. 
* Later placed as a synonym of Pholidophyllum tubulatum. 
4 Lindstrom — Geol. Mag., 1871, VIII, pp. 125-26. 
