Distribution: — A. {A.) achaeiis vvas described from Karachi, West coast of India, but 

 bas since been recorded from Aden and the Persian Gulf. If my surmise of the various species, 

 mentioned above, being synonyms of A. {A.) achaeus is correct, it would be seen that the 

 species has a wide distribution in the Indo-Pacific région. 



The "Siboga" Expédition collected a good séries of spécimens of A. (A.) achaetis from 

 Kawa Bay, West Coast of Ceram (St. 179), and two young shells at Ambon Anchorage (St. 231). 



Genus Placenta Retzius. 



1788. Placenta Retzius, Dissert. sist. nov. test. Gen., p. 15. 



1848. Placenta and Placnna Gray, Proc. Zool. Soc. London, p. 114. 



1850. Placima and Placenta Gray, Cat. Conch. Brit. Mus., I, pp. 2, 3. 



1895. Placenta and Epliippimn Dali, Trans. Wagner Free Inst. Philadelphia, p. 774. 



The différence between the shells of the so-called gênera Placenta and Ephippimn Roding- 

 (= Placitna of authors ^) are not so important as to allow of their being considered as distinct 

 gênera; they may at most be regarded as two subgenera of the genus Placenta. 



Placenta s. s., is not represented in the "Siboga" collection, but there is a single shell 

 of the species P. sella which is the type-form of the subgenus Ephippitim. 



For an- account of the subgenus Ephippmiu référence may be made to Dall's work. 



I. Placenta {Ephippiunt) sella (Gmelin). 



1756. Ostreum amboiniciun Lesser, Testaceo-Theologia, pp. 470, 472, fig. 136. 



1758. Ephippiuiii poloniciun Seba, Mus. (ex parte), p. 181, pi. XC (central fig.). 



1767. Selle Polonaise Davilla, Cat., I, p. 280, N" 582. 



1770. Selle Polonaise Knorr, Vergniigungen der Augen, etc., V, pi. XXXIX, fig. i. 



1785. Ephippiuin anglicjun wa'.t'/w//;;/, etc., Chemnitz, Conch. -Cab., VIII, p. m, pi. LXXIX, fig.714. 



1788. Placitna ephippium Retzius, Diss. Sist. nova test. Gen. p. 16. 



1790. Anomia sella Gmelin, Syst. Nat., (éd. XIII), p. 3345. 



1797. Placuna ephippium Humphrey, Mus. Calonnianum, p. 45, N° 827. 



1805. Placuna sella De Roissy in Buffon Hist. Nat. Moll, VI, p. 238. 



18 17. Anoniia sella Dillwyn, Descr. Cat., I, p. 297. 



1817. Placenta ephippium Schumacher, Essai Nouv. Syst., p. 113 (not the figure). 



18 19. Placuna sella Lamarck, Hist. Nat. Anim. sans Vertèb., VI, p. 224. 



1825. Placima sella Sowerby, Cat. Tankerville, p. 28. 



1830. Placuna sella Menke, Synop. Method. Moll., p. 97. 



1836. Placuna sella Deshayes, in Lamarck Anim. sans Vertèb., VII, p. 207. 



1842. Placima sella Reichenbach, Die Land-, Siissw., Seeconch., p. 91, pi. LVII, fig. Ç)']^). 



1844. Placuna sella Potiez & Michaud, Gai. Douai, II, p. 43. 



1848. Placenta sella Gray, Proc. Zool. Soc. London, p. 114. 



1850. Placuna sella Gray, Cat. Conch. Brit. Mus., I, p. 2. 



1855. Placuna sella Berge, Conchylienbuch, p. 90, pi. XIII, fig. 3. 



1856. Placuna sella Hanley, Rec. Biv. Shells, p. 311. 

 1860. Placuna sella Reeve, Elem. Conch., II, p. 50. 



1862. Placuna sella Bronn, Klass. Ord. Weicht., III, Atlas I, pi. XLI, fig. 7. 



1867. Placuna sella Mitchell, Cat. Mus. Madras, p. 75. 



1871. Placuna sella Sowerby in Reeve Conch. Icon., XVIII, Placuna fig. l. 



l) For a detailed discussion on the nomenclature of the family see Dai.L, op. cit. p. 769 — 774 (1895). 



