ORDOVICIAN BRYOZOA FROM THE LLANDEILO LIMESTONE 
Autozooecia curve out gradually from the branch axis in the 
endozone and change direction abruptly in the exozone to meet the 
colony surface at 90°. Autozooecia within the endozone all have 
very thin walls. 
The exozone is wide, with an average diameter of 1.9 mm. It is 
recognised by a thickening of the zooecial walls and a simultaneous 
change in zooecial orientation. Autozooecia all originate in the 
endozone where they are rounded in transverse section, becoming 
irregularly rounded in the exozone as seen in tangential sections of 
the branches. Autozooecial diameters average 0.24 mm by 0.3 mm 
within the exozone. 
Diaphragms are very abundant and closely spaced along the 
whole length of the autozooecia. They are spaced on average 0.23 
mm apart in the endozone, decreasing to 0.09 mm apart within the 
exozone. All diaphragms are basal and are orally deflected at their 
junctions with the zooecial walls. In the mid exozone of specimen 
PD 8325 there is a large interval (0.34 mm) between two adjacent 
diaphragms, which is found in the same position throughout the 
colony. The first diaphragms on the distal side of this interval are 
greatly deflected orally. In the majority of the colony, growth 
resumes as normal after the interval; however, in some small sec- 
tions the thickened exozonal wall terminates and is replaced by one 
much thinner. 
Mesozooecia are present, although not abundant, and have a 
maximum diameter averaging 0.12 mm. They originate in the 
exozone, are oval in shape in shallow tangential sections, and 
contain abundant orally deflected basal diaphragms, spaced on 
average 0.05 mm apart. 
Acanthostyles are large and abundant, with an average diameter 
of 0.05 mm and density of 14 per mm?. They originate throughout 
the exozone, commonly extending the entire length of the exozone, 
and can slightly indent the zooecial apertures. The acanthostyles are 
composed of a hyaline core surrounded by steeply dipping conical 
laminae. 
Autozooecial wall thickness averages 0.08 mm in the exozone. 
Wall microstructure is composed of inclined U-shaped laminae. 
Zooecial boundaries are indistinct. Some zooecia (especially 
mesozooecia) are infilled with laminar calcite close to the zoarial 
surface; in longitudinal section this infilling consists of broad U- 
shaped laminae. 
REMARKS. This species is characterised by the ramose colony 
form, thin autozooecial walls and rounded apertures in shallow 
tangential section. Oval mesozooecia are present and originate in the 
outer endozone/inner exozone. Basal diaphragms are abundant 
throughout the colony, and large acanthostyles are abundant in the 
exozone. 
Three species of Amplexopora have been previously described 
from the Welsh Basin. All were described by Ross (1963, 1965) from 
the Hoar Edge Limestone, Hoar Edge Group (Caradoc), Evenwood 
Quarry, Shropshire, and all vary markedly from the species de- 
scribed herein. A. thomasi Ross, 1963 is a bifoliate species with 
131 
small acanthostyles and lacking diaphragms in the endozone. A? 
evenensis Ross, 1965 and Amplexopora? sp. A of Ross, 1965 both 
have crenulate walls, diaphragms confined to the exozone and small 
acanthostyles. 
The specimens of Amplexopora from Clog-y-fran are similar to A. 
septosa (Ulrich, 1879) (redescribed by Boardman 1960) from the 
Fairview Formation (Ashgill), Covington, Kentucky. The major 
differences in A. septosa are the absence of diaphragms in the 
endozone and the presence of numerous short, off-set acanthostyles, 
as well as the long acanthostyles which occur throughout the exozone. 
Off-set acanthostyles can be identified in specimen PD 8326, but 
have not been recognised in PD 8325. 
Other examples of Amplexopora containing abundant diaphragms 
in the endozone have been described by Brown & Daly (1985) from 
the Dillsboro Formation (Cincinnati Series) of SE Indiana; they 
were identified as A. cf. septosa. Numerous specimens (over 150) of 
A. septosa were collected from this formation, including a few 
atypical specimens with abundantly spaced diaphragms in the 
endozone. Brown & Daly (1985: 24) suggested that because the 
specimens were similar in all other respects to A. septosa the 
differences may be due to environmental factors. The specimens 
from Clog-y-fran are very similar to those from the Dillsboro 
Formation, except that the short acanthostyles are less common and 
the diaphragms more abundant. 
Genus HALLOPORINA Bassler, 1911 
Halloporina cf. crenulata (Ulrich, 1893) Fig. 27 
MATERIAL. NHM PD 8315, 8394. 
DESCRIPTION. Zoaria erect with cylindrical branches, on average 
4.5 mm in diameter. 
Autozooecia are parallel to the branch axis within the endozone 
and then curve outwards gradually in the exozone to meet the zoarial 
surface at 70°. The autozooecia within the endozone have very thin 
wavy walls. 
The exozone is narrow with an average diameter of 0.76 mm. It is 
recognisable by a thickening of the zooecial walls and a simultane- 
ous change in zooecial orientation. Autozooecia all originate in the 
endozone where they are polygonal-rounded in transverse section, 
becoming oval-rounded in the exozone, as seen in tangential sec- 
tions of branches. Autozooecial diameter averages 0.18 mm by 0.22 
mm within the exozone. Basal diaphragms are rare or even wholly 
absent in the autozooecia and, if present, only one or two are found 
in the exozone. They are all deflected orally at their junctions with 
the zooecial walls and their laminae are continuous with the 
autozooecial linings. 
Exilazooecia are present and originate in the exozone. They are 
rounded in shape in shallow tangential sections, with a maximum 
diameter averaging 0.08 mm. They occasionally contain orally 
Figs 23, 24 Monotrypa sp. 23, NHM PD 8330; longitudinal section, x30. 24, NHM PD 8329; tangential section, x30. 
Fig. 25 Amplexopora sp., NHM PD 8325; longitudinal section, x12. 
Fig. 26 Amplexopora sp., NHM PD 8325; 26a, longitudinal section, showing large interval between adjacent diaphragms, x35; 26b, transverse section, x12; 
26c, tangential section, x30. 
Fig. 27 Halloporina cf. crenulata (Ulrich 1893), NHM PD 8315; 27a, longitudinal section, x22; 27b, tangential section, x61. 
Figs 28, 29 Graptodictya bonnemai Bassler 1911. 28, NHM PD 8392b, longitudinal section, x30. 29, NHM PD 8389; 29a, transverse section, x55; 29b, 
tangential section, x55. 
Fig. 30 Pushkinella sp., NHM PD 8376; 30a, tangential section, x22; 30b, tangential section, x53. 
Fig. 31 Phylloporina sp., NHM PD 8384; 31a, tangential section, x12; 31b, tangential section, x22. 
