Neithea vicinalis Stoyanow, 1949
STOYANOW, A. A. 1949. Lower Cretaceous stratigraphy in southeastern Arizona. Geological Society of America Memoirs, 38, 169 p., pls. 1-26 [p. 91, pl. 11, figs. 6, 7]
1949 Neithea vicinalis Stoyanow, 1949
A. A. Stoyanow, 1949, plate 11.
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«This small, subtrigonal and nearly equilateral shell, represented in my collection only by one specimen of right valve, belongs in that division of Neithea quinquecostata (Sowerby) group which includes the species with tertiary ribs and smooth areas.
Accordingly, the right valve has six prominent primary ribs with major interspaces between every two of them. On the inward side of the anterior (the first) and posterior (the sixth), and on the outward side of the second and fifth ribs, there are strong tertiary ribs set close to the corresponding primaries though they remain as individual ribs to the very apical part of the shell. On the inward side of the second and fifth, and on both sides of the third and fourth primary ribs, the tertiary ribs are somewhat less individualized in the ventral part of the valve where they are more or less flat and appear to be composed of two or more longitudinal striae. Dorsally, however, they pass into somewhat stronger single ribs. In the middle part of the valve, between the second and fifth primaries, there are two secondary ribs in the major interspaces between every two primaries. These ribs are narrow and strong in the apical part of the specimen but become wider and flatter ventrally. In the ventral part of the valve they are composed of from two to four longitudinal striae. Here, however, their appearance partially depends on the preservation of the shell surface. Anterior to the second, and posterior to the fifth primaries, the secondary ribs are well rounded and more elevated in the ventral region. The total number of secondary ribs is ten. The deepest intercostal depressions, or interstices, observed on the specimen, are between and on the sides of the secondary ribs. They seem to be as deep as the secondary ribs are elevated but are narrower than the latter. The ears are small and trigonal. On the anterior area there are very irregular longitudinal wrinkles which cannot be interpreted as ribs. The posterior area is entirely smooth. Except for areas, the surface of the specimen is covered with fine, minute, and closely set striae which run concentrically with the margin of the valve. In geological age, general appearance, and smooth nature of the areas, this species is closer to Neithea morrisi (Pictet and Renevier) from Hythe Beds of Lympne, England, (Woods, 1903, p. 201, PI. 39, figs, 11a-11c ) than to any other described European Neithea. From this form of the Lower Greensand the Arizona species differs in the more trigonal outline of the shell, more inflated right valve, and less prominent secondary and tertiary ribs. An examination of the original picture of "Janira morrisi" Pictet and Renevier (1858, PI. 19, figs. 2a-2d) shows a form with a rather coarse costation. Gillet (1924, p. 53, fig. 30) mentions five [?] primary and four intermediate ribs in Neithea morrisi (Pictet and Renevier) although in the accompanying illustration there are three minor ribs between two primaries. The only American species of the Neithea quinquecostata group with regular tertiary ribs and smooth areas are those originally described as Pecten quadricostatus var. Roemer (non Sowerby) (Roemer, 1852, p. 64, PI. 8, figs. 4a-4c), Neithea occidentalis Conrad (1857, p. 150, PI. 5, figs, 1a-1c), and Vola irregularis Böse (1910, p. 97, PI. 15, figs. 10-18). [25] As seen from Roemer's illustrations, in Neithea quadricostata var. the general arrangement of ribs is of the same order as in Neithea vicinalis. The right valve of the Texan species, however, is notably attenuated in the apical portion, and its ribs diverge rather rapidly toward the ventral margin which gives the entire shell a subpentagonal and not trigonal outline. Neithea occidentalis Conrad, as illustrated by Meek, is remarkably similar to the Arizona form. It even seems that the secondaries between the third and fourth primary ribs are flattened in the same degree and also split into striae in the ventral region, and that the major interspace between those primary ribs is narrow, contrary to all other related species except Neithea morrisi (Pictet and Renevier, part) and Neithea irregularis Böse. Unfortunately, the type of Neithea occidentalis Conrad preserved in the U. S. National Museum is so different from Meek's illustration as to cause reasonable doubt that Meek had that specimen in view as an example for his drawing. Pecten quadricostatus var. Roemer has been identified with Conrad's species by Conrad (1855, p. 269; 1857, 150) and Adkins (1928, p. 126). It should be noted in this connection that Neithea occidentalis, as figured by Meek in the Emory's Boundary Report, is of a more trigonal outline and has straighter ribs, less divergent ventrally. Neithea irregularis (Böse) has more prominent secondary ribs than the Arizona species. If the relation in size between the primary and minor ribs in these two species is taken into consideration, the difference is quite striking. It also appears from the illustrations of Böse (1910, PI. 15, figs. 10-18), Kniker (1918, PI. 2, figs. 1-6), and Adkins and Winton (1919, PI. 11, figs. 11-15) that the primary ribs of Neithea irregularis, especially the second and the fifth, either remain straight to the ventral margin, or are but slightly curved, which probably justifies Kniker's placing of Böse's species in the Neithea alpina (d'Orbigny) group, whereas the characters of Neithea vicinalis, i.e. the deeper major interspaces and curved outer ribs, connect it rather strongly with Neithea morrisi (Pictet and Renevier). The difference in arrangement of the tertiary ribs in their relation to the primaries in the typical forms of Neithea irregularis (Adkins and Winton, 1919, p. 68) and in Neithea vicinalis is shown in the following table. TYPE: Holotype: No. 91479.
OCCURRENCE: Perilla member, division 2b, of Lowell formation, Ninety One Hills.» ALEXANDER A. STOYANOW, 1949
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