1894. PROCEEDINGS OF THE NATIONAL MUSEUM. 97 



merely the result of the same processes which developed the original 

 hinge teeth in the priouodoiit s;^ction of the Paleopelecypoda. Where 

 the motion is purely to and fro, giving rectilinear friction of the opposed 

 surfaces, the rugiB must be parallel and regular, corresponding to the 

 direction of the movement. Where the motion may be slightly irreg- 

 ular, corresponding irregularities will appear in the rugosities. The 

 tendency of the development of rugse is to confine and limit the range 

 of motion in the interest of the safety of the mollusk, a tendency which 

 culminates in the interlocking rigid hinge of Plicatula and Spondylus. 

 Contrary to the supposition of Neumayr, I believe there is no funda- 

 mental distinction between the groups possessing Desmodont and Het- 

 erodont hinge teeth, but that both are developed according to the par- 

 ticular circumstances of the case; the immersion of the ligament and 

 development of a cartilage may occur in some genera of any natural 

 group. 



Typical species. 



GNATHODON CUNEATUS, Gray. 

 Plate VII, figs. 1 and 10. 



Gnathodon curteatus, Gray, .S<»\verby. Genera of Sh., Part x.xxvi, figs. 1-3,1831. — 

 Gray, P. Z.S. 1836, p. 104; Loiulou'sMag. N. H.,u. s.,l,p. 376, fig. 34,1838.— 

 Conrad, Medial Tertiary, No. 1, p. 23,1838.— Anton, Verz.Concb., p. 10, 1839.— 

 SowERBY, Man. Conch., 1st ed., fig. 83, 1839; 2nd ed., p. 1,54. fig. 83, 1842.— 

 SwAiNSOX, Malac., p. 370, 1840.— Kkeve, Couch. Syst., i, p. 62, pi. 43, 1841.— 

 Conrad, 2ud Bull. Nat. lust., pp. 190, 192, 1842.— De Kay, Zool. N. York, Moll., 

 p. 233, pi. 25, fig. 267, 1843.— Hanley, Descr. Cat. Kec. Sh., p. 35, pi. 10, fig. 

 22, 1843.— Gray, Gen. Moll. P. Z. S. 1847, p. 186.— Phii.ippi, Haudb. Conch., 

 p. 317, 1853.— Holmes, Post PI. Fos. S. Car., p. 41, PI. vii, fig. 10, I860.— 

 Dall, Bull. 37, U. S. Nat. Mus., p. 62, 1889. 



Rangia cyreiioides, Desmoulins, Actes Soc. Liu. de Bordeaux, v., p. 57, figs. 1-3, 

 Feb. 15, 1832.— Conrad, Am. Marine Conch, pp. 56, 57, PL xiii, 1832.— H. & 

 . A. Adams. Gen. Rec. Moll., ii, p. 380, PI. 100, figs. 4, 4a, 1856.— Conrad, Proc. 

 Acad. N. Sci. Phila. 1860, p. 232, 1861 ; Medial Tert. U. S., Index, p. 88, 1861.— 

 Prime, Proc. Bost. Soc.N. II., vii,p. 347, 1861. — Conrad, Am. Journ. Conch., 

 Ill, app., p. 30, Cat. Mactridw, 1868. — Fischer, Man. de Conchyl.,p. 1096, PI. 

 xxi,fig. 2, 1887. 



Gnathodon Grayl, Tuomey & Holmes, Pleioc. Fos. S. Car., p. 99, pi. 23, fig. 11, 

 1857; not of Conrad; IMd., Post PI. Fos., p. 41, 1860. 



Gnathodon minor, Holmes, Post PI. Fos. S. Car., p. 41, 1860; in synonymy. 



Clathrailon cuneata, (Gray Ms.) Conrad, Am. Marine Conch., p. 57, 1833; Am. 

 Journ. Sci., 1st ser., xxiii, p. 340,1833. 



Pliocene of the Carolinas and of Florida (Caloosahatchee beds). 

 Pleistocene of Cornfield Harbor, Chesapeake Bay, and Wailes' Blufi". 

 Potomac River; of South Carolina; of Florida; of the whole north coast 

 of the Gulf of Mexico and on the north coast of South America ( ?), Lea; 

 Pleistocene (!) of Matamoras, Mexico, Duges; Living in Mobile Bay, 

 Alabama, and westward on the north shore of the Gulf to Vera Cruz, 

 Mexico, in shallow water, either brackish or perfectly salt. I have re- 

 Proc, X, M. 94 7 



