OCR Text |
Show 1895.] MR. F. A. BATHER ON UINTACRINUS. 983 Here, moreover, it is to be noticed that the law stated above applies equally to U. westfalicus (fig. 3), and therefore probably is the law for the whole genus. It is unnecessary to describe the fixed secundibrachs in detail. They are flat, thin plates, with no trace of any axial canal, either in section, or superficially in the form of an axial ridge. They are about the same width as the immediately succeeding free brachials, but are much higher. In the latter respect, however, no line of demarcation can be drawn between fixed and free, since the change, though rapid, is not sudden. The fixed pinnules arising from the fixed secundibrachs unite wdth one another and with the interdistichals (or interaxdiary) and interbrachial plates to form the interdistichal and interbrachial (or interradial) areas of the dorsal cup. Each interbrachial area of an adult contains 3 pinnules on either side • and each interdistichal area contains 2 pinnules on either side (fig. 8). But only Fig. 8. Fig. 9. Fig. 8.- Fig. 9.- - Uintacrinus socialis, part of Brit. Mus. B 6527, ./, showing fixed distichals and pinnules, interdistichals, and interpinnulars. -Fixed pinnules of ditto from Brit. Mus. B 6527, q. Both figures natural size. the proximal portions of the pinnules are fixed ; the distal portions must have been freely movable and quite independent. The amount of fixation, and the number of pinnules involved, increase wdth age. A young individual, viz. q (fig. 9), shows only two pairs of pinnules in an interbrachial area; and the proximal of these appears to have only 5 ossicles fixed or modified by fixation, whereas an older individual, viz. el (fig. 10), clearly shows 9 in that state. In old individuals, even a fourth pair of pinnules m a y have been involved in the interbrachial area. The proximal fixed ossicles of these pinnules are large flat plates, both wider and higher than the succeeding free ossicles. They show no trace of any axial canal in the proximal region. The flattened plates of the fixed portions of the pinnules do not form horizontal rows across the areas, but alternate with one another, assuming a hexagonal outline. |