33 BULLETIN OF THE 



large foramen extending nearly to the posterior border. Shell structure 

 in two layers, the outer calcareous and sculptured with a peculiar netlike 

 sculpture resembling perforations ; inner layer horny, minutely tubulous, 

 as in Discina. Type Orbicula terminalis Conrad, in Nat. Hist. New York, 

 Part IV, Geology (Emmons), p. 395, fig. 4, 1842. 



I am not prepared at present to admit Siplionotreia and Acrotreta into 

 this family, but am inclined to think, with Kutorga, that they form a 

 peculiar group by themselves. 



Keyserlingia Pander (Bull. Ac. Sci. St. Petersb., Ill, p. 46, 1861, 

 Type K. reversa Pand., pi. ii, fig. 1, a-g), appears to have relations with 

 Trimerella or Gotlandia, or perhaps with Siphonolreta, but its position is at 

 present doubtful. 



Rarity of specimens and errors in identifying types are probably the 

 reasons of the confusion of forms existing in this unfortunate family. 



Genus DISCINA Lam. 



Syn. = Discina Gray, Ann. Phil., XXVI (New Scr., X), p. 244, 1825. Ibid., transl. 

 Isis, p. 494, 1834. Syn. Brit. Mus., p. 155, 1840 * and p. 88, 1842. 



— Davidson, Int. Class. Brach., pp. 51, 126, 1853/- — Woodward, 

 Man., p. 237, 1 854. — Davidson, Mem. Soc. Lin. Norm., X, p. 84, tabic, 

 p. 232, 1856. — H. and A. Adams, Gen. Rec. Moll., II, p. 584, 1858. — 

 Mrs. Gray's Moll., IV, p. 202, 1859. — Suess, Wohns., I, p. 42 (224), 

 1859. — Carpenter, Lect. Moll., Smithsonian Rep., p. 276, 1860. — 

 Hall, 13th Regent's Rep., p. 77, 1861. — Bronx, Klass. Oidn. Thicrr., 

 Ill, 1st Abtb., p. 301, 1S62. — Hall, 14th Regent's Rep., p. 130, 1864. 



— Pal. N. Y. Vol. IV, Part I, p. 15, 1870. 



=- Discina Lamarck, Hist. An. s. Vert., VI, p. 236, 1819; Ed. II, VII, p. 

 296, 1836. — Rang, Man. Moll., p. 263, 1829. — Cuvier, Regne An. 

 Ed. Voigt, III, p. 602.* — Thomas Brown, Conch. Textb., Ed. V, 

 p. 108, 1839. — Macgillivray, Ibid., Ed. IX, p. 124, n. d. 



= Crania /3 Schumacher, Essai, p. 102, 1817. 



= Crania (sp.) Gould, Moll. U. S. Expl. Exped., p. 465, 1852. 



= Orbicula Sowerby, Min. Conch., VI, p. 4, pi. 506, 1830. — Desii ayes, 

 Enc. Me'th. vers, II, tab. accph., 1830 (not Ibid., Ill, p. 668, 1832). — 

 G. B. Sowerby, Jr., Conch. Man., Ed. II, p. 209, 1842. — Morris, 

 Cat. Brit, foss., p. 123, 1843. — King, An. Mag. Nat. Hist., XVIII, 

 p. 28, 1846. — Sby., Thes., I, p. 365, 1847. — Mcerch, Cat. Yoldi, p. 

 64, 1852. — Owen, Anat. Invert., p. 503, 1855. — Chenu, Man. 

 Conchyl., II, p. 231, 1862. 



= Orbicula (sp.) Lamarck, Hist. An. s. Vert., Ed. II, pp. 317, 318, 1836.— 

 Eichwald, Urwelt Rossi., II, p. 76, 1842. 

 ;=--< Orbicula Lamarck, Phil. Zool. (Ed. 1830, p. 317), Ed. I, 1809.* — Sower- 

 by, Gen. Sh. fasc. XIII, n. d. (1821). Trans. Lin. Soc, XIII, p. 466, 



