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Megadesmus
Classification
Phylum:
Mollusca
Class:
Bivalvia
Subclass:
Anomalodesmata
Order:
Pholadomyoida
Superfamily:
Pholadomyacea
Family:
Megadesmidae
Formal Genus Name and Reference:
Megadesmus SOWERBY, 1839, p. 14
Type Species:
M. globosus, SD WOODWARD, 1854, p. 262
Images
(Click to enlarge in a new window)
Fig. F8, 4 a-c. *M. globosus (Sowerby), L.Perm. (Allandale F.), Harper's Hill, Hunter Valley, New S. Wales, 4a, LV view of shells, X0.5 (Sowerby, 1838), 4 b, dorsal view of lectotype, X0.5 (Runnegar), 4c, RV ext., X0.5 (805a).-Fig. F8,4d. M. gryphoides de Koninck, E.Australia, latex cast of interior showing that front of dorsal margin slightly overlaps that of LV, ant. part of which fits into shallow groove in front of LV tooth, X0.5 (Runnegar, n).
Synonyms
Pachydomus, Cleobis
Geographic Distribution
Australia-Tasmania-India-Argentina
Age Range
Beginning Stage in Treatise Usage:
Perm.
Beginning International Stage:
Asselian
Fraction Up In Beginning Stage:
0
Beginning Date:
298.89
Ending Stage in Treatise Usage:
Perm.
Ending International Stage:
Changhsingian
Fraction Up In Ending Stage:
100
Ending Date:
251.9
Description
Large equivalve, asymmetrical shells ornamented with coarse to medium imbricate growth lamellae and fine to medium concentric ribs; with sporadic obscure radial costae; micro-ornament consisting of radial rows of minute papillae, hinge with dental process in RV produced by fold in dorsal anterior valve margin, and socket in LV resulting from de- pression of valve edge; ligament external, opisthodetic, parivincular, supported by strong nymphs set in well-defined escutcheon; adductors subequal, the anterior smaller; 2 pedal muscle scars (protractor and retractor) situated above anterior ad- ductor, and single retractor adjoining posterior adductor; third retractor scar in and on anterior side of umbonal cavity; pallial line simple, radially striated; shell inside pallial line commonly pitted by insertions of mantle; shell and ligament of 3 layers: periostracum, outer lamellar layer and in- ner (?nacreous) layer (Runnegar, 1965).
References
Museum or Author Information
Classification
Phylum:
Mollusca
Class:
Bivalvia
Subclass:
Anomalodesmata
Order:
Pholadomyoida
Superfamily:
Pholadomyacea
Family:
Megadesmidae
Formal Genus Name and Reference:
Megadesmus SOWERBY, 1839, p. 14
Type Species:
M. globosus, SD WOODWARD, 1854, p. 262
Images
(Click to enlarge in a new window)
Fig. F8, 4 a-c. *M. globosus (Sowerby), L.Perm. (Allandale F.), Harper's Hill, Hunter Valley, New S. Wales, 4a, LV view of shells, X0.5 (Sowerby, 1838), 4 b, dorsal view of lectotype, X0.5 (Runnegar), 4c, RV ext., X0.5 (805a).-Fig. F8,4d. M. gryphoides de Koninck, E.Australia, latex cast of interior showing that front of dorsal margin slightly overlaps that of LV, ant. part of which fits into shallow groove in front of LV tooth, X0.5 (Runnegar, n).
Synonyms
Pachydomus, Cleobis
Geographic Distribution
Australia-Tasmania-India-Argentina
Age Range
Beginning Stage in Treatise Usage:
Perm.
Beginning International Stage:
Asselian
Fraction Up In Beginning Stage:
0
Beginning Date:
298.89
Ending Stage in Treatise Usage:
Perm.
Ending International Stage:
Changhsingian
Fraction Up In Ending Stage:
100
Ending Date:
251.9
Description
Large equivalve, asymmetrical shells ornamented with coarse to medium imbricate growth lamellae and fine to medium concentric ribs; with sporadic obscure radial costae; micro-ornament consisting of radial rows of minute papillae, hinge with dental process in RV produced by fold in dorsal anterior valve margin, and socket in LV resulting from de- pression of valve edge; ligament external, opisthodetic, parivincular, supported by strong nymphs set in well-defined escutcheon; adductors subequal, the anterior smaller; 2 pedal muscle scars (protractor and retractor) situated above anterior ad- ductor, and single retractor adjoining posterior adductor; third retractor scar in and on anterior side of umbonal cavity; pallial line simple, radially striated; shell inside pallial line commonly pitted by insertions of mantle; shell and ligament of 3 layers: periostracum, outer lamellar layer and in- ner (?nacreous) layer (Runnegar, 1965).