New Species Metaldetimorpha yorkei. 
The type specimen of the Ardrossan form is shown in 
Plate XXXI, Figs. 127, 127 A, C, D, E. The stone con- 
taining the fossil shows four facets A, B, C, D, and 
enlargements of facets, A, C, D, are shown in Figs. 
127 A, C, D; very little is visible on facet B. C shows 
a section which appears to be roughly transverse. A 
cavily appears in this section, but it is doubtful if this 
represents an actual inner wall and central cavity, and 
in none of the specimens can these structures be identified 
with certainty. At bb is a partial ring of thickened tissue 
forming a wall, and it is noteworthy that this is sand- 
wiched between zones of trabecular tissue both externally 
and internally. At aa a defined plate or wall occurs and 
is seen on both facets A and B; this plate does not show 
obvious pores; the elements of the trabecular mesh 
anastomiose with it. A second similar plate occurs on 
facet A. "These plates do not correspondent in character 
with the porous wall bb previously referred to. A 
section of one of these plates is seen further enlarged 
in Fig. 127 E. If the outer surface is represented at the 
upper part of Fig. 127 A it. has a very ragged character. 
Another somewhat similar fragment is shown in Fig. 
128. Here the tissue in the upper part of the figure has 
more of a septal appearance; near the bottom is a zone 
of parallel structures on a rather smaller scale; and 
between the two is a structure somewhat resembling the 
' “wall” in the type specimen, 
Another fragment, Fig. 129, has a zone of somewhat 
septa-like structures externally, and a zone rather like 
the endotheca of Ardrossacyalhus internally, a kind of 
wall separating the two tissues. 
Two further fragments are shown in Plate XLI, Figs. 
159 A, B, C. One of these fragments is shown cut 
transversely in 159 A, and it is found that the right-hand 
lower edge of this figure corresponds to one of the 
wall-like structures such as occur at Fig. 127 Cab. This 
structure, seen in longitudinal section along the face of 
the wall in Fig. 159 B, has a regularity of structure 
which comes as rather a surprise considering the general 
irregularity of the organisms. It appears that the tis- 
sue near the wall has, in fongitudinal section, a regular 
honeycomb structure and that the wall itself is formed 
by an oriented thickening of all the walls of the honey 
comb, thus encroaching on the lumina and forming a 
porous sheet, the pores of which become progressively 
smaller towards the centre of the sheet; each pore would 
thus have an hour-glass-shape as seen in transverse sec- 
tion of the organism, The second specimen, seen in 
transverse section in Fig. 159 C, presumably shows one 
of these “walls” taking a sinuous course through the 
middle of the section, 
New Species Metaldetimorpha fiindersi. 
Obscure, poorly. preserved and, for the most part, 
unsilicified fragments occur at the “Paint Mine,” Bel- 
tana, which bear some resemblance to the last described 
species, and which we place in the same genus under 
the name M. įlindersi. The type specimen is figured 
in Plates XXXII, XXXIII, Figs. 130 A—F. A view of 
32 
a complete face of the fossil is seen in Fig. 130 A. In 
this three patches of septal structure occur, with inter- 
vening patches of trabecular structure; at the left of the 
section are two curved, more or less concentric, zones 
of trabecular tissue. Very little indication is seen of 
any defined symmetrical form. The septa are incon- 
stant in their orientation, as is seen better in Fig. 130 B; 
which is taken from another face of the type specimen; 
they also vary in scale, reaching 14 mm. or more apart 
in 150 B and under 1 mm. in 130 F; note the same 
feature in another fragment 130 G, where the coarse 
structure at the right contrasts with the finer structure 
at the left, and compare this with Fig. 128 of M. yorkei, 
with coarse structure above and fine below. Wall-like 
structures occur within the septal and trabecular tissue; 
one of these is seen in section at the middle left of Fig. 
130 F, and in surface view at the top of the same figure; 
similar walls are seen in another fragment from the 
Paint Mine, Fig. 130 G, which may be compared with 
Fig. 159 C from Ardrossan. The concentric zones ot 
trabecular tissue are shown in Figs. 130 C and D in 
transverse section, also in Fig. 130 E where the lower 
part of the tissue is running longitudinally. 
The relationship of the zoned tissue to the remainder 
of the specimen is not yet clear; it may be an essential 
part of the organism, or it may be a parasitic growth 
such as we suggest the next described specimens to be 
under the name of Exocyathus. In Fig. 131 A, B is 
figured another fossil from the Paint Mine, in which 
zones of Exocyathus-like tissue form a joint coating to 
specimens of Protopharetra and Beltanacyathus. 
Apart from the fact that none of the fragments we 
have so far found either at Ardrossan or the Paint 
Mine indicate any definite general shape, Metaldetimor- 
pha has more resemblance to Metacyathus taylori than 
lo any other Archaeo we have yet seen; and we have 
found at the Ajax one irregular specimen which at first 
we considered as a fragment of a large Metacyathus 
taylori, but which has points of resemblance to Metalde- 
timorpha flindersi. 
New Genus Exocyathus. 
One of the most difficult problems of the Archaeos is 
the true nature of the structures described by Taylor as 
“roots” and “exotheca.” The great majority of Ajax 
specimens show no such structures, and, when a limited 
supply of material was available, and the structures ap- 
peared to be associated with one or two defined species 
such as Archaeocyathus sellicksi, it was natural to re- 
gard the structures as essential parts of the particular 
species. — But we now find that these structures are of 
sporadic occurrence and are associated with a minority 
of the individual specimens of such widely separated 
types as Protopharetra, Beltanacyathus, Metacoscinus, 
Archaeocyathus and Coscinocyathus. Moreover the 
same band of investment may include within itself two 
or more totally different Archaeos, and then appears 
equally connected and related to each of the included 
organisms. Although the matter cannot yet be regarded 
as decided, the evidence appears rather to point to the 
