142 THE NAUTILUS. 



The minuter species of Amnicola may sometimes be < Elected by 

 a process similar to that given above; except that for these one does 

 not plough up the bottom, but strikes the strainer over the bottom or 

 through the weeds. A mixture of species is usually obtained, which 

 may be advantageously sorted with a reading glass. 



NOTES ON FOSSIL CALIFOKNIAN PLEUKOTOMIDAE. 



BY IRA M. BUELL. 



The very large collection of fossil forms of this group, made in the 

 Pliocene of Santa Monica, California, by Dr. Rivers, formerly 

 Curator of the Museum of the State University, has afforded the 

 writer opportunity to institute interesting comparisons between forms 

 previously classified under several subgenera of this group. The 

 collection contains over one thousand specimens of these forms, 

 hence the means to test the value of specific distinctions were far 

 more perfect than were apparently present when the species were 

 first described. 



Subgenus Borsonia, 



Distinguished by plication on columella. 



1. Borsonia hooveri Arn. Of 27 specimens examined, 16 hare 

 faint to obsolete columellar plication. One shows three faint ridgeti*, 

 while the rest lack the subgeneric distinction entirely. All agree in 

 outline and number of whorls with Arnold's type, but about half 

 have almost obsolete nodes on apical whorl like D. renaudi Arn., 

 which this approaches. 



2. Borsonia bartsehi Arn. Of 70 specimens studied, 20 show 

 plications faint to obselete in most individuals, one has three, and 

 one has two faint ridges on columella. About half have transverse 

 ribs on the body whorl, and the rest are marked like D. renaudi, 



* The Rivers Collection of above 100,000 specimens of fossil Californian 

 Mollusca now becomes the property of Beloit College, Beloit, Wisconsin, and 

 Pomona College, Claremont, California, half going to each of these institutions. 

 Numerous important comparative studies have been made while the entire col- 

 lection is still intact. This great collection indicates one thing with great 

 certainty and that is that the work on the San Pedro and Santa Monica fossil 

 Mollusca will have to be entirely recast. An examination of the material in 

 Fusus, Natica, and other genera, indicate a condition similar to that described 

 above for certain Pleurotomidae. 



