Spondylus ginginensis Feldtmann, 1963
FELDTMANN, F. R. 1963. Some Pelecypods from the Cretaceous Gingin Chalk Western Australia Together with Descriptions of the Principal Chalk Exposures. Journal of the Royal Society of Western Australia, 46 (4): 101-125, pls. 1-6. [p. 111, pl. 2, figs. 1, 2]
1963 Spondylus ginginensis Feldtmann, 1963
F. R. Feldtmann, 1963, plate 2.
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«Represented by four more or less imperfect specimens of united valves of varying size, three from Molecap and one from Musk's Chalk, as well as a small imperfect right valve showing the interior and wholly attached to a fragrnent of Inoceramus from the North Chalk. Dimensions are given in Table IV.
Description.— Shell fairly large, ovate, oblique, height greater than length, the Ieft valve usually highly inflated, the right valve less so, but higher and perhaps slightly wider. Attached by the right valve, usually by the umbonal part only, this part usually much produced, some specimens showing a marked gap between the umbones of the two valves. Exterior surfaces of both valves ornamented with 70 to 80 fine evenly-spaced radial threads, and rather indistinct concentric growth lamellae; flne concentric threading, more noticeable on the right valves, is also present. A number of irregularly spaced small spines are present on the dorsal half of the left valve of the paratype.
Left valve usually highly convex, the beak terminal and sharp, and curved approximately at a right angle to the commissure. Posterodorsal margin of body of shell, where joined by ear, concave, the anterior and ventral margins convex. Ears descending slightly from the umbo: posterior ear large, extending on the holotype, from the beak to 24 mm below; dorsal margin straight, distal margin concave; anterior ear smaller, exiending to 17 mm below the beak, dorsal and distal margins straight, meeting at a very obtuse angle. On the paratype the width of the posterior ear is about 8 mm. Both ears show faint concentric threading continued upwards from the body of the valve. The left valve of the holotype is ornamented with about 80 radial threads. On the other specimens the number is about 70. Right valve less convex than the left, but considerably higher and apparently slightly longer. The umbo is produced well above that of the left valve. The area of attachment is very variable in size; on the holotype the attachment is to a smooth surface of Inoceramus, on the paratype the umbonal half of the attached portion shows flne radial ribbing, the other half well marked concentric lamellae; the other two specimens do not show any area of attachment. Ornament similar to that of the left valve, except that spines appear to be absent. Remarks.— The third specimen of which the dimensions are given is much flatter than the others, its thickness being only haif the dimension of the length of the shell, whereas in the other specimens the proportion is three-quarters. It may represent a different species, but in some European species the proportions of thickness to tength appear to be variable. In the Musk's Chalk specimen, the point of greatest width is situated higher than in the other specimens. S. ginginensis appears to resemble most nearly the European species S. gibbosus d'Orbigny, particularly the specimens from the Cambridge Greensand figured by Woods (1901, Plate XX, Figs. 5-11), but the right valves of the Gingin specimens are relatively flatter, except in the Musk's Chalk specimen, and the ears of the left valves appear to be larger. Woods (1901, p. 118) states that the right valve of S. gibbosus is variable, flattened when attached by its entire surface, more convex when attached by a part only. The regularly spaced stronger ribs found on some specimens of S. gibbosus (Woods 1901, Fig. 5) are absent from the Gingin specimens.
The holotype of S. ginginensis was found in a block of chalk that had fallen from the back of the Molecap quarry. The paratype was from 2 ft 6 ins. above the base of the Gingin Chalk, at the junction of the Uintacrinus and Marsupites zones. Judging from the dark brown colour of the test, the Musk's Chalk specimen must have come from near the base of the formation at that locality. The North Chalk specimen was from the junction of the Uintacrinus and Marsupites zones. The range of the species would appear to be the Uintacrinus zone and lower half of the Marsupites zone. Four imperfect right valves, wholly attached to fragments of Inoceramus, similar to the North Chalk specimen, were found by Dr. Glenister and Mr. Balme in the Toolonga Calcilutite of the Murchison River valley at Thirindine Point and a similar fairly large specimen was obtained by Clarke and Teichert from Pillarawa Hill farther north, as well as two small ones from Meanarra HiIl south of the Murchison River.» F. R. FELDTMANN, 1963
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