GEOGRAPHIC DISTRIBUTION. xliii 



the glands into the tubular or finger-like form, is characteristic of 

 the BELOGONA SIPHONADENIA. 



The Belogona JEuadenia in the Old World extend from Japan 

 and India southward throughout the East Indies, with a few Cor- 

 asia like forms in New Guinea and the Solomons. That they are 

 chronologically a later element than the Epiphallogona is shown by 

 the fact that they are represented in the southern and southeastern 

 portion of this range by only one genus (Helicostyla'), and even this 

 is much restricted, being absent in Australia, the Louisiades and 

 New Hebrides, etc., where Epiphallogona are wll represented. On 

 the north, the mainland of Asia offered easy passage to Japan ; and 

 during a period of mild climate in high latitudes, and of elevation 

 of the Bering Sea region, the Euadenia penetrated westward to 

 America and south east to California, Mexico and South America, 

 crossing to the West Indies by way perhaps of a Yucatan-Cuba ridge 

 of elevation. 



The date of this exodus of Asiatic life we are unable now to fix ; but 

 it could hardly have been later than the beginning of the Eocene, and 

 there are good reasons for believing it earlier. At the same time, while 

 it may have been coincident with the ingress of Epiphallogona into 

 America, it was probably later ; for no Belogona reached the Caribean 

 chain (where a well differentiated group of the other tribe is uni- 

 versally represented), and its distribution eastward in South Amer- 

 ica is less great. In North America the barrier to eastward dis- 

 tribution has apparently been due to extensive inland seas in the 

 Rocky Mountain tract, and upon their disappearance to arid clima- 

 tic conditions. At all events, we now have in America several 

 sharply defined generic types : Cepolis, the peculiarities of which 

 have been evolved on Antillean soil, and which gave rise to a side 

 line of arboreal snails, Polymita, the early origin of which is shown 

 by its retention of three cusps on all teeth ; a feature now lost in 

 the other genus, some divisions of which have also assumed arboreal 

 life, with its consequent remodelling of the radula. On the main- 

 land the Mexican genus Lysinoe offers characters clearly telling of 

 ancient divergence; and this is supported by the discovery of a 

 species apparently allied to L. ghiesbreghtiiin the Puerco Group of 

 New Mexico, this Eocene horizon being below the Wasatch Group, 

 immediately above the Laramie (H. nacimientemis White, Bull. U. 

 S. Geol. Surv. no. 34, 1886, pi. 5, f. 3-7). Associated with this 

 Lysinoe in the Pueroo are Holospira and numerous fresh-water forms. 



