30 DRYMJSUS, MEXICO AND CENTRAL AMERICA. 



natural basis whatever, cannot be made ; and while there is a great 

 variety of forms among the several species, they must still be re- 

 garded as a nearly homogenous assemblage. 



The origin of the main mass of this stock may be traced to forms 

 emigrating from the adjacent portion of South America, at various 

 times, probably beginning immediately after the last interruption of 

 communication between the two oceans at the Isthmus, and contin- 

 uing to the present day. A small number of forms, including prob- 

 ably D. domim'cus, totonacus, and some of their allies, were possibly 

 derived frpm the Antilles by way of Cuba and Yucatan ; though the 

 ancestors of this fauna also were ultimately from the Spanish main. 

 That the Mexican Drymcevs species are comparatively recent intrud- 

 ers from South America, is shown by their close alliance to Colom- 

 bian forms, most of the "groups" into which the species fall being in- 

 separable from those of northern South America, and by the fact that 

 the number of these " groups " becomes much greater as we go 

 southward, from northern Mexico to Costa Eica. Were they an old 

 constituent of the Mexican fauna, we would expect peculiarly Mexican 

 types, such as occur among Helices, etc. 



In the following account of the species, I have quoted largely 

 from Dr. von Martens, whose work on this genus, in the Biologia, 

 is worthy of high praise ; though the material studied has caused me 

 to reach different results in some instances, as would be expected. 

 With the earlier great work of Crosse & Fischer, Strebel & Pfeffer's 

 Beitrage, and von Martens' Biologia, the Mexican Bulimulid fauna 

 may be considered comparatively well known ; though so numerous 

 are the local races and closely allied species that much doubtless re- 

 mains to be done. 



Mexican Groups of Drymaus. 

 a. Peristome conspicuously expanded. 



b. A broadly open rimation ; no spiral stria?. 



Group of D.josephus, p. 31. 



>'. Perforation narrow. Group of D. serperastrum, p. 33. 

 a'. Peristome but slightly or not expanded. 



b. Short-oval, no spiral striae, peristome colored. 



Group of D. tripictus, p. 



b f . Elongated, moderately opaque, and usually streaked or 

 spotted. 



