ORDOVICIAN BRYOZOA FROM THE LLANDEILO LIMESTONE 
Genus HEMIPHRAGMA Ulrich 1893 
Hemiphragma pygmaeum Bassler, 1911 Figs 4-5 
1911 Hemiphragma pygmaeum Bassler: 289, fig. 176. 
non 1970 Hemiphragma pygmaeum Bassler; Nekhorosheva: 
84; pl. vii, figs 5-6. 
SYNTYPES. NHM D 22829, D 22536; Chasmops Limestone (up- 
per Caradoc), Oland, Sweden. 
MATERIAL. NHM PD 833 1a, b. 
DESCRIPTION. Zoaria erect with quite thick cylindrical branches, 
on average 4.5 mm in diameter. 
Autozooecia curve out gradually from the branch axis to meet the 
zoarial surface at 90°. The autozooecia within the endozone have 
thick straight walls. 
The exozone has an average diameter of 1.24 mm. It is recognised 
by a thickening of the zooecial walls. Autozooecia all originate in the 
endozone, where they are rounded and occasionally petaloid in 
transverse section, becoming rounded to circular as seen in tangen- 
tial sections of branches. Autozooecial diameters average 0.19 mm 
by 0.23 mm within the exozone. Diaphragms are found throughout 
the colony but are not common. Partial diaphragms are, however, 
very abundant everywhere. They are spaced on average 0.17 mm 
apart in the endozone and 0.15 mm apart in the exozone, and tend to 
occur on alternating sides of the autozooecia. The direction of 
deflection of the laminae of the diaphragms at their junctions with 
zooecial walls cannot be distinguished. 
Mesozooecia are present, although not common, and originate in 
the outer parts of the endozone. They are rounded in tangential 
section and have an average maximum diameter of-0.1 mm. 
Mesozooecia contain orally deflected basal diaphragms, spaced on 
average 0.08 mm apart. 
Acanthostyles are large and abundant with an average diameter of 
0.05 mm and density of 23 per mm’. They originate throughout the 
colony and can indent the autozooecial apertures to produce a 
petaloid shape. In the outer exozone they are normally found in the 
centre of the thick walls. A large hyaline core is surrounded by 
steeply dipping conical laminae. 
Autozooecial wall thickness averages 0.07 mm in the exozone. 
Wall microstructure is composed of steeply inclined, U-shaped 
laminae. Adjacent zooecial wall boundaries are occupied by wide 
granular areas. The wall structure is hard to distinguish because it is 
greatly disrupted by the large acanthostyles. Frequently zooecia are 
infilled with laminar calcite close to the zoarial surface. In longitu- 
dinal section this infilling consists of broad U-shaped laminae. 
REMARKS. Hemiphragma pygmaeum was originally described 
by Bassler (1911) from the Chasmops Limestone (upper Caradoc) of 
Oland, Sweden but has not hitherto been recognised elsewhere. 
Bassler characterised the species by its ‘mushroom’-shaped colo- 
nies, thick zooecial walls, large acanthostyles throughout the colony 
and the abundant partial diaphragms. 
The specimens from Clog-y-fran are known only in section. They 
differ from the Swedish H. pygmaeum in having a straight-sided, 
121 
more erect colony form and slightly smaller acanthostyles. Also the 
mesozooecia do not appear to originate as deeply in the colony. At 
present the Welsh material is placed within H. pygmaeum and the 
differences with the Swedish material are considered to represent 
intraspecific variability until more material can be examined. 
The genus Hemiphragma has not be previously described from 
Great Britain. A second species from Clog-y-fran, Hemiphragma 
sp., 1s also described here. The two species are very different, H. 
pygaeum being ramose in form, with thick walls and large 
acanthostyles, and Hemiphragma sp. being hemispherical, with thin 
walls and ring-diaphragms. 
H. pygmaeum was described from the middle Ordovician of Pai- 
Khoi and Vaigach Island, Russia (Nekhorosheva 1970). The 
illustrations, however, show thin-walled specimens with abundant 
diaphragms and small acanthostyles; these are considered not to be 
H. pygmaeum. 
Hemiphragma sp. Fig. 6 
MATERIAL. NHM PD 8327. 
DESCRIPTION. Zoaria large and hemispherical, on average 2.5 mm 
in diameter. Autozooecia all originate from the basal lamina and 
curve outwards towards the zoarial surface. Autozooecial walls are 
straight throughout the colony; there is no differentiation between 
endozone and exozone. Autozooecia are large with an average 
diameter of 0.39 mm x 0.47 mm and are polygonal/rounded to 
rounded in transverse section throughout the colony. Thin dia- 
phragms and ring diaphragms are present in all zooecia, spaced 0.62 
mm apart in the endozone and 0.36 mm in the exozone. These are 
basal diaphragms which are orally-deflected at their junctions with 
the zooecial walls and have laminae continuous with the autozooecial 
linings. 
Possible acanthostyles occur in the outer parts of the colony, but 
are rare. Autozooecial wall thickness averages 0.02 mm at the 
periphery of the colony. It is not possible to identify the microstruc- 
ture from available peels and thin sections. 
REMARKS. ‘The specimen described herein is characterised by a 
hemispherical colony shape. Autozooecia have straight, thin walls 
throughout the zoarium, and the autozooecial apertures are polygo- 
nal-rounded to rounded in transverse section. Diaphragms and ring 
diaphragms are present in all autozooecia. 
Bassler (1911: 282, fig. 170) described and illustrated specimens 
of H. tenuimurale Ulrich, 1893 from the type locality, the 
Clitambonites and Nematopora Beds, Lower Trenton, Minnesota 
and Iowa; and from the Wassalem Beds (Caradoc), Uxnorm, Esto- 
nia. The Welsh specimen is similar to the Estonian material: it has 
thin walls and mesozooecia are lacking, but differs in having a 
hemispherical rather than ramose colony, diaphragms within the 
endozone, and by the presence of ring diaphragms rather than partial 
diaphragms. H. tenuimurale described by Ulrich (1893) from Min- 
nesota (see Bassler, 1911: fig. 171) has fewer diaphragms in the 
endozone compared to the Estonian material. Brown (1965) des- 
cribed specimens of H. tenuimurale from the Logana and Jessamine 
Figs 2-3 Dittopora sanclerensis sp. noy. 2, NHM PD 8338 (holotype); 2a, longitudinal section, x22; 2b, transverse section, x22; 2e, tangential section, x53; 
2d, tangential section, showing infilled zooecia, x53. 3, NHM PD 8333 (paratype); longitudinal section, showing infilled autozooecia, x37. 
Figs 4-5 Hemiphragma pygmaeum Bassler, 1911. 4, NHM PD 833 1a; longitudinal section, showing partial diaphragms, x35. 5, NHM PD 8331b; 5a, 
tangential section, x35; 5b, tangential section, x94. 
Fig. 6 Hemiphragma sp., NHM PD 8327; 6a, longitudinal section, x23; 6b, longitudinal section, showing ring diaphragms, x37; 6c, tangential section, x30. 
Fig. 7 Heterotrypa sp., NHM PD 8314; 7a, longitudinal section, x26; 7b, longitudinal section, x47; 7c, transverse section, showing infilled zooecia in the 
exozone, x37; 7d, tangential section, x70. 
