External and internal structures of ovicells in Fenestella sp. cf. F. fanata, Fenestella sp. cf. F. delicatula and Hemitrypa hibernica are described from the silicified Carboniferous limestones of northwest Ireland. Although on structural grounds it is evident that these ovicells have much in common with those of cyclostome genera it is not possible to discern much more than a superficial resemblance to corresponding features in cheilostomatous bryozoans. This strengthens the affinities between the cyclostomes, cryptostomes and trepostomes, but shows no relationship between these orders and the Cheilostomata.