The skeletons of most living Polycycstina are covered with a veneer of very fine, particulate opal that imparts a smooth finish to specimens viewed by the scanning electron microscope. In contrast, the opaline skeletal surfaces of Polycystina preserved in Miocene and older sediments have slightly "etched" appearances, with small dissolution pits presumably representing the removal of the finer particulate opal and revealing a substratum of opaline microspherules approximately 1 to 3 µ in diameter. The cross-sectional surfaces of broken spine bars on the vast majority of specimens have a somewhat conchoidal fracture, but otherwise they are smooth and give no indication of internal

End_Page 471------------------------------

microspheralic ordering.

We have observed three specimens of heavy-walled Polycystina from the Miocene of California and Eocene sediments of the Norwegian Sea that appear to be in a more advanced stage of dissolution. They display a highly ordered microspherulitic internal composition. After extensive observations of many specimens, we conclude that this structure is not unique to these three specimens and may be characteristic of robust polycystine skeletal elements in general.

Cross-sectional views of broken lattice bars and spines are composed of as many as 34 concentric lamellae consisting of beadlike strings of microspherules. The microspherules are oblong with their long axes parallel to the radius of the skeletal element, producing lamallae of about 0.5 µ thick. Lamellar thickness if remarkably uniform in both Spumullaria and Nassellaria. Dissolution of less resistant lamellae results in delamination, which may be followed by flaking of loose segments.

End_of_Article - Last_Page 472------------