NO. 2 PYRAMIDELLID MOLLUSKS — BARTSCH 55 



To show that this mixup in Cerion is not unusual in the wilds of 

 nature, I may report that in 1912, while in the Bahamas with the 

 expedition of the Marine Biological Laboratory of the Carnegie Insti- 

 tution, we made a trip through South Bight to the flamingo colony 

 on the west coast of Andros Island. On this trip we found many 

 fingerlike extensions of low ridges of land into South Bight. Each 

 of these spits harbored an abundant colony of Cerions resembling in 

 a general way C. viaregis. When 100 of each of these colonies were 

 biometrically measured, each one showed a distinctive curve. The 

 most startling Cerion feature, however, was a colony of a magnificent 

 huge white species to which I gave the name Cerion mayori, in honor 

 of Dr. Alfred G. Mayor, the director of the expedition. 



Returning to this region in 1921, I met an altogether unexpected 

 state of affairs. The region had been swept by a hurricane anl floods, 

 and the Cerions had been carried inland and dumped on the ridges 

 in masses, all mixed up, a wonderful opportunity for hybridization 

 of compatible elements. 



A problem of this kind in fresh-water mollusks is going on at the 

 very doorsteps of the Nation's Capital. When I began gathering 

 mollusks in and about the District of Columbia to prepare a check 

 list of its fauna I was greatly surprised to find Goniobasis virginica 

 a most variable assemblage. It seemed that there were scarcely two 

 individuals in my collections that were exactly alike. In size they 

 varied from dwarfs to giants as i to 5, and in sculpture their variation 

 ranged from smooth to axially ribbed and spirally Urate. These 

 combinations of sculpture might be constant on all the postnuclear 

 whorls of an individual or indiscriminately varied in the same shell. 

 The color, too, might be unicolor or spirally banded. 



Extending my collecting down the valley of the Potom.ac to where 

 the salt-water influence inhibited Goniobasis from existing, I found 

 that shortly below the mouth of the Occoquan Creek all the shells 

 were uniformly multihrate. If one had only such a collection he 

 would not hesitate to call it Goniobasis mulfilirata. 



Going north and up the Shenandoah Valley and also in Occoquan 

 Creek above the falls at Occoquan, I found that Goniobasis had 

 smooth shells usually with a spiral brown band. 



In the Potomac itself I found an interesting state of affairs. The 

 Chesapeake and Ohio Canal contained a fluxed fauna like that at 

 Washington, and the same was true on the shoreline of the Maryland 

 side of the river, but on the Virginia side below the mouth of the 

 Shenandoah the smooth form prevailed a long way down, becoming 



