1921] NATURAL SCIENCES OF PHILADELPHIA. 357 



Eocene of the Gulf or mid-American littoral. The D. septetnden- 

 tatus of Gabb, a Lower Claibornian species of Texas, corresponds 

 perfectly with what, on theoretical grounds, the earliest form of the 

 genus should be. 



It w^ould seem that the groups Ranularia, Lampusia, and Dis- 

 torsio, diverged about this time from a common stock. D. septem- 

 dentatum is but slightlj^ distorted. The irregular bulging of the 

 whorls, correllated with the rise and fall of the sutures and conse- 

 quent alternate widening and contraction of the body chamber, 

 has just begun, and is yet far less strongly developed than in any 

 later species. The spreading callus, which masks the front in re- 

 cent forms, has not appeared. Theteeth are small, and the mouth 

 not contracted in a grimace. The sculptural features of Distorsio 

 are present, but the earlier whorls retain those of other small Eo- 

 cene Tritons. 



Acceleration along the lines foreshadowed in D. septemdentatus 

 rapidly ensued, typical Distorsios appearing in the Vicksburg Oli- 

 gocene. We may suppose that the group spread westward through 

 the open strait between the Americas, and the swimming Sinusigera- 

 like larvae, caught in the Equatorial current, were carried to the 

 West Indies. In southern Europe the genus appeared in the middle 

 Miocene, but whether derived from America or the eastern seas is 

 doubtful. 



Of the recent species, D. anus (L.),the type of Distorsio, has 

 strongly defined characters, everywhere recognized. The others 

 have suffered great vicissitudes in nomenclature, and in order to 

 determine the status of the Oligocene forms a revision of the specific 

 nomenclature was undertaken. The following names and probably 

 some others, have been proposed in the group: 



1807 Distortrix reticulata Link, Beschr. Rostock Samml, 122. 



1809 Murex cancellinus Lam., Ann. du Mus. ii, p. 225. 



1811 Distorla acuta Perry, Conchology, pi. 10, fig. 1. 



1817 Murex mulus Dillwyn, Catal. ii, p. 704. 



1821 Murex lortuosus Borson, Mem. Reale Accad. Sci. Torino, xxvi, p. 306, 

 pi. 1. fig.4. 



1822 Triton clathratus Lam., Anim. s. Vert, ix, p. 186. 



1829 Triton personatum M. de Serres, Geogn. Terr. Tert., p. 118, pi. 3, fig. 



11, 12. 



1833 Triton constrictus Broderip, P. Z. S., p. 5. 



1844 Triton ridens Reeve, Conch. Icon., fig. 46. 



1844 Triton decipiens Reeve, 1. c. fig. 102. 



1848 Triton crassidens Conrad, Jour. Acad. Nat. Sci. I. 118, pi. 11, fig. 40. 



1849 Triton simillimus Sowerby, Quart. Jour. Geol. Soc, vi, 48. 

 18.52 Triton subclathratum d'Ofbigny, Prodr. Pal. Fr., Ill, 77, 175. 

 1860 Distorsio pusilla Pease, P. Z. S. p. 397. 



