Statistical Methods and Techniques for the Measurement of Average Index Ratios of Ostracod Carapaces

Abstract

Height-Length Index ratios were measured with different methods, the projection method and the photographic method. The ostracod material, available for the measurements, consisted of the living ostracocd Cypricercus sp. and the Pliocene Cytheridea pannonica Mehes from the Vienna Basin. Histograms show and explain the homogeneity of the available material and ground plans detect the inclusion of various molt stages. Possibilities of applications are mentionied. Historical outline of the problem.-More than one hundred years ago A. Reuss published a monograph under the title Die fossilen Entomostraceen des oesterreichischen Tertiaerbeckens (The Fossil Entomostraceae of the Austrian Tertiary Basin). The book is still in use although many corrections have been made recently on the taxonomy, for which the shape, the markings of the carapace, and the pattern of the muscle scars are most important. Cooper, Sohn, Scott, Kesling and others discovered that the presence of early molt stages in the material caused confusion in describing new species and even new genera. Height/length measurements and determination of the height/length index ratio were introduced in the early forties of this century, and, their use increased rapidly since. Measurements of the length and of the height of the valves disclose considerable variation in size and shape within one and the same species, but the height /length ratio or form ratio, expressed as an index number, eliminates the various sizes and limits attention to the roundness or the elongation of the valves. The normal distribution curve, or histogram, may still show a surprisingly great variety due to the inclusion of mutations and of material from different environments. Since measurements with the micrometer under the microscope are very time-consuming and hard on the eyes, easier techniques were devised after long experimentation. TECHNIIQUES OF CARAPACE MEASUREMENT Preparation of ostracod material for measurement.-Measurement of single valves is relatively easy, since they usually lie flat. A piece of strong cardboard is glued on a microscope glass slide with two to five circular holes punched in the central part of the cardboard. The specimens are placed in the holes with their contact margin resting on the glass slide. A cover -glass is inserted into the cardboard, which is split for this purpose at the layer opposite the glass slide. Thus the specimens are enclosed between the glass slide at the base and the cover glass on top, permitting microscopic observations in both reflected and transmitted light. No more carapaces should be placed in one cell than can easily lie flat on the glass. Most octracodes are fossilized with both valves closed and cannot be placed with the fairly flat conltact margin on the glass slide. If investigation of these in transmitted light is required, we must embed them in plastic or gum tragacanth with the con-tact margin accurately oriented parallel to the slide. This work, however, requires skill. Mounting the specimens on slides coated with paraffin is easier, but is possible only, if reflected light is sufficient. Living ostracod material has to be treated with bleaching peroxide, in order