Neithea georgetownensis Kniker, 1919
KNIKER, H. T. 1919. Comanchean and Cretaceous Pectinidae of Texas. Bulletin of the University of Texas, 1817: 1-56, pls. 1-10 [p. 31, pl. 6, figs. 1-3]
1919 Neithea georgetownensis Kniker, 1919
H. T. Kniker, 1919, plate 6.
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«Dimensions.— Large specimen: Height 46 mm.; length 40 mm.; breadth 16 mm. Small specimen: Height 34.5 mm.; length 29 mm.; breadth 11 mm.
Description.— Shell medium to large, solid, subtrigonal, higher than long, globose, subequilateral. Ears small and triangular. Ventral margin semicircular, and polygonal in outline. Postero-dorsal margin appreciably longer than antero-dorsal. The right valve is strongly convex and has a prominent incurved umbo that passes beyond the short hinge line. As in the other forms of this group, there are six prominent radiating ribs, the anterior and posterior being slightly smaller. In each one of the level depressions there are two lower ribs. The ribs, especially the prominent ones, are exceedingly strong in this species. In this form there are present compound ribs, each having a number of elevations and depressions on top. In addition to this, there is a fine rib in each intercostal depression, extending from the umbo to the base. The prominent ribs are high, broad, and round, while those in the depressions are also broad, but low, and flattened considerably. The latter usually have one main depression in the middle of the top and a smaller one on each side. On a few ribs a riblet is in the middle, but this seems to be an exception. Occasionally the main depression is somewhat to the side, and there have been found two secondary depressions on one side. The prominent ribs are wider than the others, and, as a rule, have one or two more depressions on their upper portion. Usually two of the furrows on top are more or less equal. On account of the fine ribs in the intercostal spaces, these latter are wide, often approximating the ribs in width, or even surpassing them occasionally, especially in the anterior and posterior groups of ribs. As a rule, they are somewhat narrower. On the areas there usually are from one to four fine ribs. On both valves, on the areas, and on the ears are found delicate concentric lines of such fineness that they are barely visible to the naked eye. Since at the base the convexities of one valve fit into the concavities of the other, there is not so very much difference in width between ribs and intercostal spaces in the left valve. The fine ribs inthe spaces of the right valve are usually to one side. Consequently the main furrow on the ribs of the left valve is in the same position. Near the base of the slope there is often a secondary furrow corresponding to a small rib far down on the slope of a rib of the right valve. In the depressions there is usually one principal rib and a few secondary ones. This valve is slightly concave and usually has a few secondary ribs on the areas. Remarks. — Only a very well preserved specimen shows the characteristics as described above. In a weathered shell the prominent ribs of the right valve are rounded and have a few faint radial lines impressed upon them. A cross-section is dome-shaped. The lower ribs show a sinus down the center. There are also variations in the ornamentation. Occasionally there is not a complete set of fine ribs inthe intercostal spaces, and sometimes some of the secondary sinuses on the ribs are missing. Affinities. — In Neithea budensis n. sp. it is found that the prominent ribs are made up of a number of secondary ones, and that the two in the depressions are always plain. In N. georgetowensis the secondary ribs on the prominent ones are not really so prominent as those in the form mentioned above, The two species are, furthermore, distinguished by the split ribs in the depressions and the fine ribs in the interspaces of both valves of N. georgetownensis, which are not found in N. budensis. Moreover, the antero-dorsal and postero-dorsal margins of N. georgetownensis are shorter and, consequently, the ventral margin longer than is the case in N. budensis. This causes them to differ in shape. In N. texana there also are rounded prominent ribs, and flat ones in the depressions. But these differ from the ribs of N. georgetownensis inbeing- plain, wider, and closer together. The shape of the two species is entirely different, N. georgetownensis in being more convex and more elongate than N. texana. Besides, the raised ribs of the former species are much more prominent and the ventral margin more sinuous than is the case in the latter. Number of specimens: 20 Occurrence: Georgetown Limestone, Austin, Texas» HEDWIG THUSNELDA KNIKER, 1919
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