128 
Family RHABDOMESIDAE Vine, 1884a 
Genus RHABDOMESON Young & Young, 1874 
TYPE SPECIES. Millepora gracilis Phillips, 1841, by monotypy, 
from the Devonian of north Devon, England (for discussion relating 
to the problems with this type species see Wyse Jackson & Bancroft 
1995a, 1995b). 
Rhabdomeson progracile Wyse Jackson & Bancroft, 1995 
Fig. 16 
Fig. 16 Rhabdomeson progracile Wyse Jackson & Bancroft 1995. Upper 
part of the Glencar Limestone (Viséan, Asbian), Carrick Lough, County 
Fermanagh; BMNH PD9473 (paratype); 16a, typical zoarial fragment 
showing cylindrical shape of branch, with the spiral arrangement of 
autozooecia in curved interlocking rows; autozooecial apertures are oval 
to elliptical in shape; one large acanthostyle is placed distally of 
apertures, x25; 16b, detail of 16a, x130. 
Fig. 17 Rhabdomeson rhombiferum (Phillips, 1836). Upper part of the 
Glencar Limestone (Viséan, Asbian), Carrick Lough, County 
Fermanagh; BMNH PD9485; 17a, growing tip of branch showing 
cylindrical colony form; autozooecia are arranged in longitudinal and 
obliquely intersecting rows; apertures are oval in shape, and narrow 
slightly distally; short blunt stylets surround each autozooecial aperture, 
x50; 17b, detail of 17a showing autozooecial aperture surrounded by 
stylets, x130. 
P.N. WYSE JACKSON | 
MATERIAL. BMNH PD9473-9484, TCD.34026-34028, BELUM | 
K3095, Upper part of the Glencar Limestone (Viséan, Asbian), 
Carrick Lough, County Fermanagh, Ireland. TCD.42520, Upper | 
part of the Glencar Limestone (Viséan, Asbian), Sillees River, } 
} 
{ 
County Fermanagh. 
DESCRIPTION. Zoariaare dendroid, with cylindrical branches rang- 
ing in diameter from 0.61 to 1.07mm. There may be some increase 
in branch diameter prior to, or subsequent to, lateral branch develop- 
ment. Bifurcation is rare. The longest zoarial fragment examined | 
measures 8.1mm in length. 
Autozooecia are budded from a straight hollow cylindrical axis _ 
0.14 to 0.29mm in diameter, in an annular or spiral pattern. In thin 
section autozooecial chambers are triangular to pentagonal in shape 
when seen in transverse section. Vestibules are orientated at a high 
angle to the zoarial surface. Acanthostyles arise as rods of granular 
calcite in the lower portions of the exozone. Interchamber endozonal 
walls are 0.1mm in width and are composed of an inner granular 
layer surrounded by a fine laminated skeleton. 
Autozooecial apertures are pyriform to oval in shape, and moder - 
ate to small in size. They are crowded or arranged in quincunx in 14 - 
to 18 longitudinal rows around the branch. Interapertural spacing is 
greatest longitudinally where apertures are spaced one diameter | 
apart and up to 5 in a 2mm line. Transversely adjacent apertures are | 
spaced less than one diameter apart. Autozooecial apertural dimen- | 
sions and spacing are approximately constant in each branch! 
fragment. However, some considerable differences are found be- | 
tween zoarial fragments. 
A large acanthostyle, up to 0.12mm in height, is always found i) 
distal to autozooecial apertures. Rare zoaria bear only this single 
acanthostyle (Fig. 16b); more frequently one or two smaller: 
acanthostyles lie proximal to the first in a longitudinal line between } 
adjacent autozooecial apertures. Acanthostyles are usually abraded, 
and appear as faint protruberances on the zoarial surface. 
DISCUSSION. A complete systematic description of R. progracile’ 
is given in Wyse Jackson & Bancroft (1995a). 
Rhabdomeson rhombiferum (Phillips, 1836) Fig. 17 
MATERIAL. BMNH PD9485-9506; TCD.34029-34036, 42591b, 
stone (Viséan, Asbian), Carrick Lough, County Fermanagh. Ireland; 
TCD.42521-42524, Upper part of the Glencar Limestone (Viséan, 
Asbian), Sillees River, County Fermanagh. 
DESCRIPTION. The dendroid zoarium is composed of irregularly 
bifurcating delicate branches, with a polygonal or circular cross-)_ 
section, that range in diameter from 0.41 to 0.86mm. Branch width i 
remains approximately constant along their entire lengths. Branch: 
ing either by bifurcation or development of lateral branches at a high 
angle of between 68° and 90° from parent branch. There is ne 
increase in branch diameter prior to or subsequent to branch devel 
opment. In no specimen was there more than one bifurcation 0 L. 
lateral branch observed. 
Autozooecial apertures are moderate to large in size, pyriform tt)’ 
oval or ellipsoidal in shape, and are arranged in quincunx in eight te} 
eleven longitudinal rows around branches. 
Apertural size, shape and spacing is very variable around thi 
branch. A distinct barren area 0.25mm in width, with four longitudi) 
nal rows of small acanthostyles, is found on some branches and ca’ 
be regarded as delineating the branch reverse surface. In all zoanl 
the autozooecial apertures are long, thin, and oval in shape on thi 
reverse surface. Towards the obverse surface apertures becom|}) 
