Abstract

Three modern representatives from the Order Allogromiida sensu lato (Myxotheca sp., Cribrothalammina alba, and Hyperammina sp.) were prepared for ultra-structural observations using high pressure freezing followed by freeze substitution, methods that greatly improve the quality of fixation of both the cell body of smaller Foraminifera and the test. The three taxa examined illustrate three different types of test construction. Myxotheca sp. has an organic test characterized by a distinct herringbone pattern, a feature found in several other monothalamous Foraminifera. Cribrothalammina alba, along with several other saccammininds, has a flex-ible test that is in contact with the cell body and consists of a finely fibrous IOL (inner organic lining) and prominent agglutinated layer. Though Hyperammina sp. also has an IOL and prominent agglutinated layer, the IOL consists of a thin band of crescent-shaped fibers and thus differs from that found in C. alba and similar saccamminids. Such ultrastructural differences may reflect differences in the mode of growth of these two taxa: C. alba grows by expanding its single-chambered test whereas Hyperammina grows by lengthening a conical test. Foraminiferal systematics has been based almost exclusively on test composition and structure. Identifying the general themes of test fine structure and construction in the otherwise morphologically simple allogromiids should contribute to a better understanding of systematic relationships within the group.

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