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Exogyra
Classification
Phylum:
Mollusca
Class:
None
Subclass:
Pteriomorphia
Order:
Pterioida
Suborder:
Ostreina (oysters)
Superfamily:
Ostreacea
Family:
Gryphaeidae
Subfamily:
Exogyrinae - TRIBE Exogyrini
Formal Genus Name and Reference:
Exogyra SAY, 1820, p. 43
Type Species:
E. costata; M
Images
(Click to enlarge in a new window)
Synonyms
Exegyra, Exogira, Exagyra, Exogera, Exoyra, Costagyra, Fluctogyra, Nutogyra
Geographic Distribution
N.Am.(Gulf Mexico area-Atl. Coastal Plain-Utah)-S. Eu.-N. Afr.-Angola-Nigeria-W. Asia-India
Age Range
Beginning Stage in Treatise Usage:
Cret.
Beginning International Stage:
Berriasian
Fraction Up In Beginning Stage:
0
Beginning Date:
143.1
Ending Stage in Treatise Usage:
Cret.
Ending International Stage:
Maastrichtian
Fraction Up In Ending Stage:
100
Ending Date:
66.04
Description
Small to large, mostly medium-sized (largest diameter of largest known specimen 21 cm.). Shell very inequivalve, RV flat to concave, LV tumid, convex, much larger than RV. Attachment area variable, large in most species, very small in few; spirally curved umbonal half of LV qmvex and rather tumid, except in species with small attachment area, spiral keel obtuse and rounded, never prominent, evanescent in old age, hardly noticeable in most individuals. LV outline orbicular to oval, margins in most species convex throughout except for dorsalmost part of posterior margin which may be rectilinear or concave; very few species somewhat falcate in outline, with much of posterior part of the LV margin gently concave (see Fluctogyra). Anterior part of RV exterior with many crowded concentric upturned growth squamae parallel to anterior valve margin; posterior part of RV with flat-lying foliaceous growth squamae. LV with wide commissural shelf delimited by rounded curb in species that lack chomata or by narrower rounded curb and adjoining shallow gutter in species that have chomata. Anterior set of LV chomata consisting of 3 mm. long straight parallel radial ridgelets separated by grooves, all closely spaced; posterior set consisting of 7 mm. long, transverse, vermiculate ridgelets covering entire width of commissural shelf near hinge, this part of shelf sigmoidally curved so that it forms inward and upward projecting platform covered with vermiculate chomata in some specimens (Fig. J89, J90). RV margin reflexed to form commissural shelf which allows valve to become countersunk into LV. ante0 rior margin reflexed at about 70 • Quenstedt muscle imprint, visible only on RV, narrow, elongate, and about 1 mm. removed from posterior end of resilifer. Sculpture of LV, and to lesser extent of RV, consisting of foliaceous growth squamae alone or in combination with various patterns of radial costae which include hyote spines or rows of successive transverse bulges on crests of radial costae. 9Fluctogyra VYALOV, 1936, proposed as a section of Exogyra, supposedly is characterized by vague radial undulations. Such costules are present also on E. (E.) ~rralicoslala STEPHE>lSON (1914, p. 49, pI. 15, fig. 4, pI. 16, fig. 1-2), which cannot be separated from Exogyra s.s. However, the type species of Fillctogyra differs from Exogyra 5.5. in somewhat elongate falcate outline oC LV margin, which seems an unimportant distinction.-Costagyra VYALOV, 1936, also proposed as a section of Exogyra, carries chamata and has a few projecting radial ribs on R'! (REESlDE, 1929) (Fig. )91). Although it is very close to Exogyra 5.5., it may be a valid minor 5ubdivision.--Some apparently isolated species of Exo* gyro have LV with a very small attachment area, a not so tumid, rather slender umbonal tip end, and a greater number of spiral volutions. It is puzzling that some individuals have no chomata, whereas others nave well-developed ones.]
References
Museum or Author Information
Classification
Phylum:
Mollusca
Class:
None
Subclass:
Pteriomorphia
Order:
Pterioida
Suborder:
Ostreina (oysters)
Superfamily:
Ostreacea
Family:
Gryphaeidae
Subfamily:
Exogyrinae - TRIBE Exogyrini
Formal Genus Name and Reference:
Exogyra SAY, 1820, p. 43
Type Species:
E. costata; M
Images
(Click to enlarge in a new window)Synonyms
Exegyra, Exogira, Exagyra, Exogera, Exoyra, Costagyra, Fluctogyra, Nutogyra
Geographic Distribution
N.Am.(Gulf Mexico area-Atl. Coastal Plain-Utah)-S. Eu.-N. Afr.-Angola-Nigeria-W. Asia-India
Age Range
Beginning Stage in Treatise Usage:
Cret.
Beginning International Stage:
Berriasian
Fraction Up In Beginning Stage:
0
Beginning Date:
143.1
Ending Stage in Treatise Usage:
Cret.
Ending International Stage:
Maastrichtian
Fraction Up In Ending Stage:
100
Ending Date:
66.04
Description
Small to large, mostly medium-sized (largest diameter of largest known specimen 21 cm.). Shell very inequivalve, RV flat to concave, LV tumid, convex, much larger than RV. Attachment area variable, large in most species, very small in few; spirally curved umbonal half of LV qmvex and rather tumid, except in species with small attachment area, spiral keel obtuse and rounded, never prominent, evanescent in old age, hardly noticeable in most individuals. LV outline orbicular to oval, margins in most species convex throughout except for dorsalmost part of posterior margin which may be rectilinear or concave; very few species somewhat falcate in outline, with much of posterior part of the LV margin gently concave (see Fluctogyra). Anterior part of RV exterior with many crowded concentric upturned growth squamae parallel to anterior valve margin; posterior part of RV with flat-lying foliaceous growth squamae. LV with wide commissural shelf delimited by rounded curb in species that lack chomata or by narrower rounded curb and adjoining shallow gutter in species that have chomata. Anterior set of LV chomata consisting of 3 mm. long straight parallel radial ridgelets separated by grooves, all closely spaced; posterior set consisting of 7 mm. long, transverse, vermiculate ridgelets covering entire width of commissural shelf near hinge, this part of shelf sigmoidally curved so that it forms inward and upward projecting platform covered with vermiculate chomata in some specimens (Fig. J89, J90). RV margin reflexed to form commissural shelf which allows valve to become countersunk into LV. ante0 rior margin reflexed at about 70 • Quenstedt muscle imprint, visible only on RV, narrow, elongate, and about 1 mm. removed from posterior end of resilifer. Sculpture of LV, and to lesser extent of RV, consisting of foliaceous growth squamae alone or in combination with various patterns of radial costae which include hyote spines or rows of successive transverse bulges on crests of radial costae. 9Fluctogyra VYALOV, 1936, proposed as a section of Exogyra, supposedly is characterized by vague radial undulations. Such costules are present also on E. (E.) ~rralicoslala STEPHE>lSON (1914, p. 49, pI. 15, fig. 4, pI. 16, fig. 1-2), which cannot be separated from Exogyra s.s. However, the type species of Fillctogyra differs from Exogyra 5.5. in somewhat elongate falcate outline oC LV margin, which seems an unimportant distinction.-Costagyra VYALOV, 1936, also proposed as a section of Exogyra, carries chamata and has a few projecting radial ribs on R'! (REESlDE, 1929) (Fig. )91). Although it is very close to Exogyra 5.5., it may be a valid minor 5ubdivision.--Some apparently isolated species of Exo* gyro have LV with a very small attachment area, a not so tumid, rather slender umbonal tip end, and a greater number of spiral volutions. It is puzzling that some individuals have no chomata, whereas others nave well-developed ones.]