ORDOVICIAN BRYOZOA FROM THE LLANDEILO LIMESTONE 
Autozooecia are roughly parallel to the branch axis within the 
endozone, and gradually curve outwards to meet the zoarial surface 
at approximately 70°. Walls are thin and slightly wavy within the 
endozone. 
The exozone has an average diameter of 1.6 mm, and is recog- 
nised by a slight thickening of the zooecial walls. The autozooecia 
originate within the endozone, where they are polygonal in trans- 
verse section. No tangential sections are available. Diaphragms are 
present throughout the autozooecia. They are spaced on average 
0.26 mm apart in the endozone and 0.1 mm apart in the exozone. The 
diaphragms are basal and are orally deflected at their junctions with 
the zooecial walls. Many diaphragms in the endozone are inclined 
and some are sigmoidal. Cystiphragms are numerous in the exozone 
where there are, on average, ten present per mm. The cystiphragms 
are normally restricted to one side of the autozooecia. 
Mesozooecia are rare and originate in the exozone when present, 
with an average maximum diameter of 0.08 mm. They contain 
abundant orally deflected basal diaphragms, spaced on average 0.04 
mm apart. 
Small inconspicuous acanthostyle-like structures have been ob- 
served in the exozone; tangential sections are needed for their 
precise identification. 
Autozooecial wall thickness averages 0.04 mm in the exozone. 
Wall microstructure is composed of inclined V-shaped laminae. The 
zooecial boundaries are dark and crenulated. Some zooecia are 
infilled close to the zoarial surface. In longitudinal section this 
infilling consists of broad U-shaped laminae. 
REMARKS. Only one incomplete specimen of Homotrypa has been 
found in this present study; this has made identification difficult, 
especially as no tangential section could be obtained. The specimen 
is characterised by thin autozooecial walls in the exozone and rare 
mesozooecia. The autozooecia contain abundant diaphragms, espe- 
cially in the exozone, often sigmoidal in shape in the endozone. 
Cystiphragms are extremely numerous in the exozone. 
Ross (1963, 1965) described three Ordovician species; Homo- 
trypa sp. A, Homotrypa sp. B and H. oweni from the Hoar Edge 
Group (Caradoc Series), Shropshire. Homotrypa sp. A of Ross 
(1963) has widely spaced diaphragms and cystiphragms; meso- 
zooecia and dense acanthostyles are present but not always easy to 
observe in tangential section. The autozooecial apertures are po- 
lygonal to subpolygonal in shallow tangential sections. Homotrypa 
sp. B of Ross (1963) has thin autozooecial walls within the endozone 
and subpolygonal zooecial apertures. Diaphragms are present 
throughout the colony and acanthostyles are long and thin. The 
specimen from Clog-y-fran differs from Homotrypa sp. A in having 
more abundant diaphragms and cystiphragms; and from Homotyrpa 
sp. B by the absence of long thin acanthostyles. Homotrypa oweni 
Ross, 1965 differs from the Clog-y-fran specimen in having a cone- 
shaped or encrusting colony form, mesozooecia and rare diaphragms. 
The arrangement of the cystiphragms and diaphragms in the 
Welsh specimen is similar to that found in Homotrypa similis Foord, 
1883 (e.g. Karklins 1984: pl. 5, figs 2, 3). This species is well known 
in North America and Eastern Europe. H. similis has acanthostyles 
in the exozone but in the specimen from Clog-y-fran their presence 
is questionable. Bork & Perry (1968: 1053) recognised H. similis 
(from the Guttenberg and Ion Formations, middle Ordovician, Iowa, 
USA) in longitudinal section ‘by extremely gradual curvature of the 
zooecia towards the zoarial surface, diaphragms throughout most of 
the axial region and well-developed cystiphragms and diaphragms 
in the mature zone’; the Welsh specimen fits this description. 
Karklins (1984: 29) noted a difference between specimens of H. 
similis from the Trenton Beds (middle Ordovician), Ottawa, Canada, 
129 
and the Lexington Limestone (middle/upper Ordovician), Ken- 
tucky, USA, and those from the Wassalen Beds (Caradoc) of Estonia 
(described by Bassler 1911). Specimens from Estonia have rela- 
tively broadly serrated autozooecial boundaries and well-defined 
acanthostyles which commonly indent the autozooecial apertures. 
North American specimens have narrower serrated boundaries and 
poorly-defined acanthostyles. The cystiphragms in the Estonian 
specimens are more closely spaced in the exozone that those of 
North America. Middle Ordovician specimens from Vaigach Island 
in Russia, illustrated by Astrova (1965: pl. 35), do not, however, have 
large acanthostyles, and their cystiphragms first occur in the outer 
endozone and become closely-spaced in the exozone. Thus, there 
appears to be a wide range of variation within the species H. similis. 
The Clog-y-fran specimen is compared herein with H. similis 
rather than positively identified as this species because the incom- 
plete specimen does not provide sufficient information. 
Genus MONOTRYFPA Nicholson, 1879 
Monotrypa sp. Figs 23-24 
MATERIAL. NHM PD 8329, 8330. 
DESCRIPTION. Zoaria hemispherical, on average 13.5 mm in diam- 
eter. 
The majority of autozooecia originate from the basal lamina and 
curve gently outwards to meet the zoarial surface. Autozooecial 
walls are straight throughout the colony. No differentiation between 
endozone and exozone can be recognised. The autozooecia are 
polygonal-rounded in transverse section, with an average diameter 
of 0.29 mm by 0.32 mm. Diaphragms are rare, usually only one per 
autozooecium. In one specimen (PD 8329), however, there is a small 
area of the colony with relatively numerous diaphragms which are 
thin, basal and orally deflected at their junctions with the zooecial 
walls. 
Autozooecial wall thickness averages 0.02 mm at the periphery of 
the colony. Wall microstructure is composed of inclined U-shaped 
laminae; the zooecial boundaries are indistinct. Occasionally zooecia 
are infilled with laminar calcite close to the zoarial surface. In 
longitudinal section this infilling consists of broad U-shaped laminae. 
REMARKS. The specimens described herein are characterised by 
the hemispherical colony form and polygonal-rounded autozooecia. 
Autozooecial walls are thin and straight, with no differentiation 
between endozone and exozone. Diaphragms are generally uncom- 
mon. Only two specimens are known, both in peels. 
Several species of Monotrypa with thin straight walls and sparse 
diaphragms have previously been described, e.g. M. testudiformis 
and M. cantarelloidea described by Dreyfuss (1948: pl. 2, figs 4, 5, 
8-10) from the upper Ordovician of the Montagne Noire. 
The poor quality of the Welsh specimens prevents detailed com- 
parisons with other species, so their identification is left in open 
nomenclature. 
Genus AMPLEXOPORA Ulrich, 1882 
Amplexopora sp. Figs 25-26 
MATERIAL. NHM PD 8325, 8326. 
DESCRIPTION. Zoaria erect with thick cylindrical branches, on 
average 8 mm in diameter. 
