Neithea hartmani 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. 48, pl. 10, figures 3-6, 12]
1919 Neithea hartmani Kniker, 1919
H. T. Kniker, 1919, plate 10
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«? Pecten quadricostatus Roemer, 1852, Kreidebild. v. Texas, p. 64.
Dimensions.— Height 11 mm.; length 10.4 mm.; breadth 5 mm.
Description. — Shell small, subequilateral, subtrigonal, length practically equaling height; gently curved base with six prominent projections formed by the extensions of the raised ribs and separated by shallow concavities; postero-dorsal margin longer than antero-dorsal, but both rather long. Greatest length below the middle. Ears medium; hinge line straight.
Right valve moderately convex, umbo prominent and incurved. The ornamentation consists of twenty-one simple, principal, radiating ribs of which every fourth one, including the first and the last, is much stronger than the others. The ribs curve gently outward. Of the three ribs in the depressions the middle one is sometimes very slightly stronger than the other two, or two are of the same strength and an outside one is somewhat smaller. In other depressions, no difference in the strength of the ribs can be made out. When there is a difference, as in the cases mentioned above, it is so slight that it can be detected only upon close study. The prominent ribs are practically three times as wide and as high as those in the depressions, which really are very insignificant. On all of the specimens the most posterior prominent rib, which is reduced in size, has a smaller rib on the anterior side. Allribs are rounded, and the depressions, as a whole, are flat. The intercostal spaces are rounded and are as wide as, or wider than, the ribs inthe depressions, those adjoining the prominent ribs being no exception. The ribs of the first and the last group are not so strong as those in the others. At the base the six prominent ribs form blunt projections, and the depressions are bounded by gently curved, incisions. Narrow, radial, secondary ribs are present on the areas, On the posterior area there are usually five, but one specimen shows six. On the anterior area the number could not be determined definitely, due to lack of preservation of these areas. But indications are that secondary ribs are just as abundant here as on the other area. Although the ears are poorly preserved, the posterior one of one specimen clearly shows radial ribs. These have the same shape and arrangement as those on the areas and seem to be but a continuation of them. Fine, crowded, concentric ridges further ornament this valve, including the areas and ears. The left valve is flat or slightly concave, and is ornamented with prominent radiating ribs that correspond to the intercostal spaces of the right valve. The same general manner of arrangement of ribs is found as in N. texana and allied forms, only there are two ribs in each, depression, instead of one. There are alternately two high and two low ribs. The ribs are rounded, subequal, and generally are wider than the interspaces. Of course, the furrow separating each pair of raised ribs is an exception to this and is considerably wider than the ribs. Concentric ridges are present as on the right valve. The areas and ears are missing. Remarks. — One right valve shows only two ribs in one of the depressions, and on one left valve there is a small rib in one of the broad furrows. Whether these are abnormalities or variations is an open question. Stoliczka (22, p. 438) has found all manners of gradations between N. quadricostata (Sow.) and N. quinquescostata (Sow.) of the Southern Indian Cretaceous. There is a possibility that a similar condition exists inthe Austin Chalk between N. hartmani n. sp. and N. casteeli n. sp. More extensive collecting in different portions of the State is necessary in order to determine this.
Affinities.— D'Orbigny (8. p. 644, pl. 447, figs. 1-7) and many writers after him have described a Senonian form as Neithea quadricostata. Later Pictet and Campiche (17, p. 253) showed that d'Orbigny's species is distinct from Sowerby's and named the former N. faujasi. Choffat (2, p. 149) believes N. faujasi to be identical with regularis Schl. D'Orbigny's species differs from Sowerby's in being smaller, in having much smaller ears and a shorter hinge-line; in the anterior and posterior areas sloping outward only slightly; in a smaller apical angle; in having seven or eight ribs on the areas, instead of a smaller number like the true quadricostata; in the six. prominent ribs being of a lesser elevation and, consequently, the groups less clearly demarcated; and inhaving the greatest length of the shell below the middle.
It remains to be shown how N. hartmani n. sp. differs from similar forms of Senonian age. N. hartmani n. sp. differs from N. quadricostata (d'Orb.) chiefly in its shape, which is not so long and less globose; in its more angular base; in its stronger raised ribs; and in having longer antero-dorsal and postero-dorsal margins and more secondary ribs on the areas. Zittel (26, p. 115, pl. 18, fig. 4) describes an Upper Cretaceous form as N. quadricostata. It differs from the Texas species in being higher, less angular at the base, and in having only two or three secondary ribs on the areas, or none at all. Moreover, Zittel says, six of the ribs on the left valve are characterized by greater strength. This certainly is not the case with the Texas specimens, where alternate pairs are raised. Judging from Zittel's figures, the arrangement of ribs on the left valve of his form, is entirely different from that of the left valve of N. hartmani n. sp. Another quadricostata of the Upper Cretaceous is White's (23, p. 37, pl. 4, figs. 1-2) form from Brazil (province Sergipe.). It is higher than N. hartmani, and has no secondary ribs on the areas. Furthermore, judging from White's pictures, there is less difference in size between the prominent ribs and those in the depressions, than in the Texas species. The intercostal spaces in N. hartmani seem to be wider. In conclusion it ought tobe mentioned that N. hartmani varies from the Upper Cretaceous quadricostata form, as generally described, in having radial ribs on the ears and only five or six ribs on the areas, instead of seven or eight, and having the prominent ribs at least three times as strong as those in the depression, instead of nearly the same size. As has been pointed out above, other Upper Cretaceous forms have only two or three ribs on the areas, or none at all. Number of specimens: 12.
Occurrence: Upper layers of the Austin Chalk, Walnut Creek, Sprinkle, Texas.»
HEDWIG THUSNELDA KNIKER, 1919
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