On the genus Pompholyx and its Allies. 333 



at the same time, they present opportunities of ascertaining the 

 variation which species are liable to, if any, in time. In the pre- 

 sent case there has been no variation since the deposition of 

 this stratum, now 71 feet below the surface, but the accumula- 

 tion of subjacent strata may have been very rapid, so that these 

 forms may be extremely recent. At all events the examination 

 is of interest, and it is to be hoped that similar collections will be 

 examined in the same manner, as, if no other immediate result is 

 arrived at, yet, at least, contributions to a knowledge of the local 

 distribution of these minute organisms will be obtained, and this 

 is a point upon which it is desirable to obtain data, from its 

 bearing upon geology, in which branch of science the diatoma- 

 ceae have already made their mark. 



XXVII. — On the genus Pompholyx and its allies, imth a re- 

 r/'sy'on of the Leun^eidje of authors. 



By William H. Dall. 

 Read March 14, 1870. 



The receipt, through the kindness of Mr. Harford, of a num- 

 ber of specimens of Pompholyx enables me to correcl an error 

 into which I had been led, and to add something to our knowl- 

 edge of this singular mollusk. 



In 1866, Mi-. William M. Gabb furnished me with drawings 

 and descriptions of the .-oft parts of the animal of /'. effusa, 

 Lea, taken from life In both, it was represented a- possessing 

 two pair of eyes ; one pair at the inner basis of tin- tentacles 

 (as in Physd) and one at the tips of the tentacle- (as in 

 Vertigd). 



On this point Mr. (iabb was positive, and, reiving on In- 

 accuracy, I separate'! this mollusk from the other fresh water 

 pnlmonates, provisionally, in a subfamily by itself. 



