Neithea theodori 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. 34, pl. 6, figs. 6-9]
1919 Neithea theodori Kniker, 1919
H. T. Kniker, 1919, plate 6.
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«Dimensions. —Large specimen: Height 33 mm.; length 31 mm.; breadth 14 mm. Small specimen: Height 27 mm.; length 25.5 mm.; breadth 9 mm.
Description.— Shell medium to large, almost equilateral, broad, height practically equaling length, polygonal outline, ventral margin sub-semicircular with six definite angles. Ears missing. Right valve convex; umbo gently incurved. On the surface are found sixteen plain, principal, radiating ribs. Every third one is raised above the others high enough to be distinctly recognized on first sight. These prominent ribs are rounded, while those in the depressions are low,and flat on top. Allhave rather steep sides. The depressions as a whole, as well as the intercostal spaces, are flat or flattened. The raised ribs, ifat all, are only slightly wider than the others. The intercostal spaces approach the ribs in width. In some specimens the most posterior rib is a double one, while in others the anterior one shows this characteristic. Secondary ribs are distributed on the areas in an irregular manner. Sometimes there is none, whereas on other areas there is one or as many as three or four. On the two areas of the same specimen, the same number is not always found. Fine concentric lines are present on the body of the shell, the areas, and the ears. Remarks. — In the above description it was mentioned that the ribs are plain. There are, however, a number of specimens that show a slight variation from this condition; namely, split ribs. Since there is a gradual transition from the plain ribbed specimens to the ornamented ones, and since those specimens which show the split ribs in the greatest abundance do not show this characteristic to a very great extent, the writer does not feel justified in classing the specimens having split ribs as a distinct variety. No difference inshape, and proportions of the shell, and relative size of the ribs can be detected in the two forms. The prominent ribs are hardly ever affected, but occasionally a few show a very faint sinus. If only a few ribs on the shell show the furrow, these are located in the middle and never near the anterior and posterior borders. As itis, most specimens show the splitting to a very limited extent, but there are a few specimens in which practically all the ribs have a shallow groove on top. The left valve has sixteen promninet flat-topped ribs. These are plain, low, and not very much narrower than the intercostal spaces. In the spaces there is an extremely fine' rib, whenever there is, corresponding to the former, a split rib inthe right valve. Affinities.— There is considerable likeness between N. theodori and N. georgetownensis and its variety subirregularis. The typical specimens of the former resemble more the variety of the latter, and the two split forms are more or less alike. But N. georgetownensis and its variety are elongate forms, whereas N. theodori is broad and short. In the latter species the prominent ribs are only slightly wider and not much higher than those in the depressions, and there are no fine ribs inthe intercostal spaces, while in N. georgetownensis and its variety there are very conspicuous raised ribs that are much higher and wider than the others, and fine ribs in the spaces. The split ribbed forms of N. theodori differ from N. georgetowensis primarily in the shape of the shell and relative size and proportions of the ribs, the differences having been pointed out above. Furthermore, there are no ribs in the interspaces and the manner of splitting is much more simple inthe former than in the latter form. This species differs from N. texana in its shape, which is more globose; in the greater difference in height between low and raised ribs; in its narrower ribs;and inits more sinuous ventral margin. Number of specimens: About 175. Occurrence: Georgetown Limestone, Austin, Texas.» HEDWIG THUSNELDA KNIKER, 1919
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