83 



torn must have been searched each day by hungry fishes. Not- 

 withstanding this condition of things, certain si)ecies occurred in 

 considerable numbers. Such as have the curious habit of remain- 

 ing motionless in the presence of enemies, and such as burrow- 

 readily in mud, were very common in some of the pools. Con- 

 sequently, when it is said that invertebrate animals were not com- 

 mon in tliese waters, it is meant that, as compared with permanent 

 lakes elsewhere, there was not here a great diversity of forms,, 

 represented each by an abundance of individuals. 



MOLLUSCA. (Shell Fish.) 



The Mollusca of the locality belong to two groups, — those with 

 a spiral shell of a single piece (Gastropoda), represented by the 

 river snails, and tliose with a shell of two halves hinged together 

 along the back (Lamellibranchiata), represented by the clams. 

 Both groups furnish important items of fish-food, and both were 

 well represented in the pools. 



The snails were nearly all of small size, none of those seen hav- 

 ing shells over 1.25 inches in length. These creatures are well 

 suited to a residence in these ponds. Some of them at least can 

 breathe either in water or in air, and hence can travel to other 

 pools if the water dries up. A part of them never need 

 to do this, for when the pools dry up, either in winter or 

 summer, they resort to the mud and rubbish of the exposed 

 bottom, close up their shells, and remain inactive till the water 

 comes again. They are ordinarily seen creeping about over the 

 bottom, where they feed upon the microscopic plants and ani- 

 mals or upon decaying organic matter in the form of a slimy 

 coat on sticks and mud. If pressed with hunger, they have been 

 known to resort to animal food, and in some instances devour their 

 own kind. Some of them burrow into the mud at the bottom and 

 become torpid in winter, but more active species may be seen 

 moving over the bottom under the ice. The eggs are laid in 

 spring attached in masses to sticks and dead leaves. The young, 

 hatch in two or three weeks, according to temperature. 



Family Limn^id.e. (Pond Snails.) 



Phijsa ancillaria, Say. 



{Physa ancillaria, Say, Jour. Acad. Nat. Sci. Phil., v, 124, 1825.) 



Common in Long Lake and in Willow Slough. The largest ex- 

 amples taken measure about .50 inch in length, 



Phjjsa heterostropha, Say. 



{Limnea heierosirojiha. Say, Am. ed. Nich. Enc, pi. i, f. 6, 

 1817, 1818, 1819, [as cited by BinneyJ.) 



This was probably the most common snail in the bottom-land 

 pools. It is one of the thin- shelled species, with about four 

 whorls, and differs from the preceding in having a longer and 



