ACADEMY OF SCIENCES. 93 



The local distribution of species, is more interesting to naturalists on this 

 coast. On account of the peculiarity of our wet and dry seasons, in which 

 latter nine-tenths of the surface-water dries up in this State, it is not generally 

 so well adapted for the concentration of many species in one locality as east of 

 the Mississippi. Most of our waters, therefore, furnish but few species, and so 

 isolated are they in the dry season that many local forms are generated by the 

 influence of local causes. 



The most notable exception so far discovered is near the junction of the Sac- 

 ramento and San Joaquin rivers, where, by the discoveries of Messrs. Carlton, 

 Rowell, and Dunn, we are informed of the occurrence of no less than twenty- 

 one univalves and five bivalves in the fresh water. This point, as we well 

 know, receives the flow of waters from a vast range of country extending from 

 Lat. 35° to 42^, or about 500 miles, with a width of over 100. Doubtless 

 there is no other point on the coast where so many can be found, and by extend- 

 ing the region as far as Sacramento and Stockton five or six more forms may 

 be added, all of which will probably be yet detected near the former place. 



At Mountain Lake, near San Francisco, a sheet of permanent and clear 

 water, shallow but cool at all seasons, we find only seven forms of univalves 

 and three bivalves, which is even more than I have heard of in any other so 

 limited space. At Santa Cruz, a year's search in two perpetual streams fur- 

 nished only five univalves and two bivalves, four of the seven distinct from 

 those of Mountain Lake. 



Walnut Creek, however, a little stream running N.W. from Mt. Diablo, half 

 dry in summer, supplied Mr. Yates with eight univalves and one bivalve in a 

 semi-fossil state ; and, quite remarkably, I found three of the former distinct 

 from those of the river junction, a few miles eastward, where, however, closely 

 allied forms take their place. ^j, 



Thus we find in every locality, however limited and closely joining others, a 

 peculiar group of forms, among which are usually one or more of each prom- 

 inent genus, representing those found elsewhere. This alone is strong evidence 

 that the numerous local forms of these genera are only varieties, although per- 

 haps reproduced at several distant points under similar circumstances. 



Specific Characters of the Limneidce. 



The great difficulties and differences of opinion among naturalists, as to what 

 constitutes a species, are nowhere more strikingly exhibited than in the fresh- 

 water nioUusca, on account of the variability just mentioned as connected with 

 their local conditions. Sometimes it is easy to perceive the causes of variation, 

 and to give these their due weight as mere accidental circumstances, but more 

 frequently we can only assign them to some unknown local influences. Some 

 authors, from studying only dead shells, are led to give such variations undue 

 importance; or, from^the ambition to make new species, wilfully pervert them 

 to the rank of good specific characters. 



I will briefly mention some of the characters that seem only of varietal value, 

 and those apparently really specific. The thickening at or near the edge of the 

 lip is attributed by Dr. J. E. Gray to the necessity for a stronger margin either 



