ZOOLOGY. 345 



lecting from their native seas, shores, lakes, rivers, and forests, the 

 marine, fluviatile, and terrestrial mollusks ; 00,000 of whose shelly 

 skeletons, external and internal, arc accumulated in orderly series in 

 the cabinets with which the floors of his house now groan." 



EFFECT OF PRESSURE OF THE SEA OX FISHES. 



DR. WILLIAMS has shown that a gold fish, when the water in which 

 it was placed was subjected to a pressure of four atmospheres, became 

 paralyzed. He also states the following conclusions, as deduced from 

 his own experiments : 1. That round fishes, having an air-bladder, 

 cannot, without injury, be exposed to a pressure of more than three 

 atmospheres. 2. That the use of the air-bladder is not so much to 

 regulate the specific gravity of the animal, as to resist the varying 

 force of the fluid column, and thus to protect the viscera and abdomi- 

 nal blood-vessels against excess of pressure. 3. (Though in this case 

 the results are less striking,) flat fish exhibit a limited capacity only 

 for sustaining pressure. From these observations, Dr. Williams infers 

 that the condition of pressure regulates the distribution of fishes in 

 depth. 



EXTINCTION OF SPECIES OF SHELLS IN OHIO. 



. 



AT the American Association, Cincinnati, Dr. Kirkland, of Cleve- 

 land, Ohio, stated some interesting facts, in relation to the extinction 

 of species of fresh-water shells within a comparatively recent period. 



It has been alleged that geologists are sometimes compelled to draw 

 a little on their imaginations, in order to supply time for the accom- 

 plishment of all the revolutions that seem to have taken place in the 

 structure of the earth. Within the memory of individuals now living, 

 the recent vegetable and animal kingdoms of Ohio have undergone 

 changes almost sufficient to mark a geological period. Numerous spe- 

 cies, once abundant, are now very rare, or have become extinct. 

 Other species have, in some instances, supplied their places. No class 

 has suffered more extensive and fatal changes than our mollusca. 

 Forty-one years since, when I was first acquainted with this State, 

 every durable pond, lake and river, abounded with fluviatile bivalves. 

 Ohio, at that day, probably contained a greater number of species than 

 could be found on all the globe, North America excepted. With the 

 clearing and cultivation of our lands many disappeared ; the depreda- 

 tions of swine, during low stages of water, have destroyed immense 

 numbers ; the wash of cities, manufactories, and barn-yards, is still 

 more fatal to them ; and, in several instances, epidemics have extir- 

 pated immense numbers. After the construction of our canals, many 

 of the rarer species rapidly increased for a few years, and our conchol- 

 ogists flattered themselves that these thoroughfares would preserve their 

 favorites ; but experience has shown that the accumulation of filth in 

 the canals is fatal to most of the species. A few, especially Anodon- 

 tas and Alasmodontas, continue to thrive and increase, while the finer 

 species of Uniones have perished. Ten years since, the Unio truncatus 



