252 J. p. SMITH AGE OF THE AURIFEROUS SLATES. 



Aucella concentrica, Fischer, Dr C. A. White. Monograph XIIT, U. S. 

 Geol. Survey, p. 230, pL IV, figs. 6-10. 



This species, whose affinities were first recognized by Meek, occurs in 

 great numbers in the slates at Texas ranch in all stages of compression 

 and distortion, so that specimens may be obtained showing all the varia- 

 tions figured by Meek, and many others also. 



C. A. White^ unites A. erringtoni, Gabb, with Aucella piochi, Gabb, of 

 the Knoxville group, Neocomian, and with A. pallasi, Keyserling, and 

 A. mosquensis, von Buch of the Upper Jura of Russia, and A. crassicollis, 

 Keyserling of the Lower Cretaceous, Neocomian, of Russia. But it has 

 been shown by Lahusenf that the Russian species are not all the same, 

 but are well characterized, and are found in very different horizons and 

 with different faunas, and thus themselves become in a measure index- 

 fossils. Lahusenf has represented this in a table, dividing the Russian 

 Upper Jura and Lower Cretaceous into zones on the basis of the occur- 

 rence of the various species of Aucella. Thus Aucella pallasi, Keyserling, 

 is shown to be confined to the Upper Jura and to range from the upper 

 Oxford to the lower Portland ; A. mosquensis, von Buch, ranges from 

 the middle Portland into the lower portion of the upper Portland, while 

 A. crassicollis, Keyserling, is confined to the Lower Cretaceous, Neocomian, 

 and is very similar to if not identical with A. piochi, Gabb, of the Knox- 

 ville. Aitcella concentrica, Fischer, is not recognized by Lahusen, because 

 Fischer de Waldheim described under that specific name several different 

 species from different horizons ranging from Oxford to Neocomian. It 

 therefore becomes extremely improbable that all the American species 

 of Aucella are identical with each other, since they have been based on 

 this hybrid species. Meek, in his description of A. erringtoni, Gabb,§ 

 notes the similarity to A. pallasl, Keyserling, and says they are very pos- 

 sibly identical. Lahusen, in his monograph, || also notes the resemblance 

 between left valves of A. erringtoni and A. pallasi, and on page 33 of the 

 same work he compared the right valves of ^. erringtoni and A. bronni, 

 Rouiller, the only difference between them being that on the California 

 species the radial lines are very distinct only on the anterior part of the 

 shell. But in the collection from Texas ranch are numerous specimens 

 that have the radial lines just as strong on the posterior as on the ante- 

 rior part of the shell, and some even stronger. It thus becomes probable 

 that A. erringtoni, Gabb, represents in reality the two species, A. pallasi, 

 Ke3^serling, and A. bronni, Rouiller, which, because of the state of pres- 

 ervation, we are not now able to distinguish from each other. 



* Monograph xiii, U. S. Geol. Survey, p. 230. 



tMem. Com. Geol. (Russia), 1888, vol. viii, no. 1, " Ueber Russlsche Aueeilen."' 



X Op. cit., p. 20. 



gGeol. of California, vol. i, p. 480. 



II Mem. Com. Geol., vol. viii, no. 1, p. ;^,.">, 



