The Nautilus. 



Vol. VII. OCTOBER, 1893. No. 6 



ILLUSTRATIONS OF MEXICAN MELANIANS.i 



BY H. A. PILSBRY. 



The distribution of the Melanians and of land operculates in the 

 Americas forms a most interesting chapter in zoogeography. As is 

 well known, the family Pleurocericke comprises all of the Melanians 

 found living in North America above the Rio Grande ; and more- 

 over no members of this family or subfamily are known to exist out- 

 side of this area. South of the Texas boundary there are few mela- 

 nians or none throughout northern Mexico ; but as we approach the 

 isthmusof Tehauntepec the characteristic neotropical genera Pachy- 

 chilus and Hemisinus appear, and are represented by a considerable 

 number of species and innumerable local races. The richness of 

 this fauna in varietal forms rivals the prolific streams of Tennessee 

 and northern Alabama. Every spring and stream has its peculiar 

 variations, often so distinct typically, that the naturalist is tempted 

 into extravagance in naming them as species. The illustrations here 

 with given represent some of the forms of Pachychilus glaphyrtis 

 from the State of Tabasco, sent to the Academy of Sciences by 

 Professor Rovirosa, a zealous and enlightened naturalist of that 

 State. 



1 Reprinted by permission from the Pioc. Aead. Nat. Sci., Phila. 



