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Eastonia (Merope) Nicobarica, Gmel. 
Mactra rugosa Indiae orientalis, CHemMNnitz, Conchyl. Cabin., VI, 1782, p. 238, pl. 24, fig. 237. 
— nicobarica, GMELIN, Syst. Nat. ed. XIII, 1790, p. 3261, No. 24. 
Anatinella Nicobarica, GMELIN, TENNANT, Natural History of Ceylon, 1861, p. 389. 
Lutraria (Merope) Nicobarica, GMELIN, v. MARTENS, Journ. Linnean Society, Zoology, 1887, vol. 21, p. 217, 
pl. 16, fig. 7. 
= -- nicobarica, GMELIN, THURSTON, Ramésvaram Island and Fauna of the Gulf of Manaar, 
2 ed., 1895, p. 128. 
Mactra Aegyptiaca, CHEMNITZ, Conchyl. Cabin., XI, 1795, p. 218, pl. 200, figs. 1955—56. 
Lutraria Egyptiaca, Gray, Haney, Catalogue of rec. biv. shells, p. 26, pl. 6, fig. 34. 
Mactra A‘gyptiaca, CHEMNITZ, REEVE, Conchol. icon., VIII, 1854, Mactra, Sp. 112. 
Lutraria Aegiptiaca, — IssEL, Malacologia d. Mar Rosso, 1869, p. 52. 
Merope gyptiaca, Gray, ANGAS, Proc. Zool. Soc. London, 1871, p. 100. 
-—  Aegyptiaca, CHEMNITZ, DUNKER, Index molluscor. mar. Japon., 1882, p. 184. 
Eastonia — aegyptiaca, — Pitsspry, Marine Mollusks of Japan, 1895, p. 120. 
Standella Aigyptiaca, CHEMNITZ, SHOPLAND, Proc. Malacol. Soc. London, V, p. 177. 
Mactra reticulata, SPENGLER, Skrivter af Naturhistorie Selskabet, V Bd., 2 Hefte, 1802, p.119. 
Lutraria Chemnitzti, Puivipri, Zeitschr. f. Malakozool., 1849, p. 26, No. 99. 
Between Koh Riot and Koh Mesan, 3—5 fathoms, sand (‘/2). Koh Kahdat, 1 
fathom, sand (*/2). 
Long. 18—43 mm. 
Distribution: — Singapore (shallow water, Sv. Gap), Mergui Archip., Nico- 
bars, Ceylon, Karachi, Aden, Red Sea (Cuemnitz, IsseL, Forskau (Mus. Univ. Hafn.), 
STURANY). — Java, Panay Isl. (Mus. Univ. Hafn.), Japan, Liukiu Isls., Lake Mac- 
quarie (New South Wales). 
As the original specimens of CHemnitz’s “Mactra rugosa Indiae orientalis” and 
Mactra Aegyptiaca have been at my disposal, it has been possible for me to verily 
the reciprocal relation of these forms. Mactra rugosa Indiae orientalis has been 
described by Cuemnirz from a specimen in SPENGLER’s collection, and was after- 
wards named Mactra reticulata by the latter. The figure given by CHEMNITz is very 
bad, but REEveE’s figure, Sp. 112, is an excellent illustration of the species, both as 
regards its form and its sculpture; the reticulata-form thus indicates the individuals 
with pointed posterior end and coarser radiating ribs, and the interstices between 
the ribs 2—8 times the width of the ribs. The other species M. Aegyptiaca has 
been figured fairly well by Cuemnitz; it comprises the forms with oval shells, and 
finer, more closely set radiating ribs; but between these two extremes there is an 
extensive series of intermediate forms, which I have before me, and which distinctly 
show that it is same species which varies. I have also at hand that small characteristic 
form which Ep. v. Martens describes and figures from “King Island, mud-bank ex- 
posed at spring-tide;” my specimen was taken at Singapore in shallow water, it is 
only a thin-valved, littoral, local form of E. Nicobarica Gm., ornamented with rather 
slight and delicate ribs. 
D. K. D. Vidensk. Selsk. Skr., 7. Reekke, naturvidensk. og mathem. Afd. V. 3. 29 
