APPENDIX TO CHAPTER V. 



REMARKS ON THE GENUS AUCELLA, WITH ESPECIAL REF- 

 ERENCE TO ITS OCCURRENCE IN CALIFORNIA. 



By Charles A. White. 



The fo.ssil shells of the geuus Aucella, although preseuting no features which 

 especially attract the attention of the ordinary observer, have come to possess 

 unusual interest in certain fields of paleontological and geological inquiry. This is 

 mainly due to the constancy of the distinguishing characteristics of the genus, its 

 wide geographical distribution, its restricted range in geological time, and the contro- 

 versy which has arisen as to the particular geological epoch which it represents. 

 During the progress of his work, the results of which are recorded in this volume, 

 these shells have become of especial iuterest to Dr. Becker because of their preva- 

 lence in certain of the strata with which he has had to deal. I have therefore, in 

 compliance with his request, prepared the following remarks upon the genus, its geo- 

 graphical distribution, probable range in geological time, and the variation of the 

 forms which have been referred to it under various specific names. 



It is well kuown to paleontologists that at least a large part of the different 

 genera which have been proposed for the Aviculida', the family to which Aucella be- 

 longs, are not so clearly definable and distinguishable from one another as could be 

 desired, and also that the forms which have beeu ranged as species under those gen- 

 era respectively are often found to be so exceedingly variable that it is difficult to 

 decide whether they ought to be treated as species or only as varieties. While the 

 features which distinguish Aucella as a genus are not so consj)icuous as those which 

 characterize many other niolluscan genera, they have been found to be very constant 

 in all the specimens yet known, even in cases of the most extreme variation in size 

 and shape of the shell. Consequently this genus has not been found to merge into 

 related generic forms by a modification of its distinguishing features, as have some of 

 the other recognized genera of the Avicnlidie, and we may speak o{ Aucella as a genus 

 with much more defiuiteness than we are able to do concerning any of the species 

 which have been recognized under it. 



The feature which more than any other distinguishes this genus being the short 

 and peculiarly infolded anterior ear, the embedding of the shells in the stony matrix 

 2-26 



