ORDOVICIAN BRYOZOA FROM THE LLANDEILO LIMESTONE 



133 



Description. Zoaria are reticulate and anastomosing, only frag- 

 mentary specimens have been found at Clog-y-fran. No exterior 

 frontal views of the colonies are available because frontal surfaces 

 are all embedded in sediment. 



Fenestrules are oval, with diameters averaging 0.78 mm by 1 .67 

 mm. Branches are rounded and average 0.48 mm diameter. The 

 reverse side of the colony is striated. 



The exozone is distinguished by a change in the orientation of the 

 autozooecia and a thickening of the zooecial walls. Rounded zooecial 

 apertures are present on the frontal side of the colony. The apertures 

 average 0.09 mm by 0.12 mm in diameter in the exozone. Three to 

 four zooecial rows occur on each branch. 



Zooecial walls are thin and straight. Occasional basal diaphragms 

 have been observed which are deflected orally at their junction with 

 the zooecial walls and in some cases have laminae continuous with 

 the zooecial linings. 



The microstructure is laminar, although hard to distinguish. Pos- 

 sible acanthostyle-like structures are present. 



Remarks. The specimen from Clog-y-fran is fragmentary and 

 embedded in sediment, which makes identification difficult. It is 

 characterised by rounded branches and oval fenestrules. Zooecial 

 walls are straight in the endozone, becoming thickened in the 

 exozone. Zooecial apertures are rounded and three to four rows 

 occur on each branch. Occasional diaphragms are present and 

 acanthostyle-like structures have been recognised. 



Phylloporina hillistensis, described from Estonia by Lavrentjeva 

 (1985: pi. iii, fig. 2), has similar thick straight endozonal walls and 

 occasional diaphragms but differs from the Clog-y-fran specimen in 

 having more abundant zooecia per branch and lacking striae on the 

 reverse of the colony. 



Acknowledgements. I would like to thank Dr. RD. Taylor and Dr. 

 J.C.W. Cope for supervising this project, which was carried out under the 

 tenure of a Natural Environment Research Council Studentship. I am grateful 

 to Dr. D.H. Evans and Mr. F. Cross for assistance in the field. I would also like 

 to thank Mrs D.G. Evans for drafting Table 1 and Fig. 1 . and Mrs L. Weaver 

 for re-typing the manuscript. 



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