DESCRIPTIONS OF UPPER JURA FOSSILS — PELECYPODA. 431 



pared with the A. concentrica as figured by Keyserling to see that they 

 are not similar, the concentrica of Fischer having a right umbo short and 

 rounded. Fischer's specimen came from Amackshak bay, near Souk- 

 trum, Alaska. 



Typical examples of this species also occur on left bank of Merced 

 river one-quarter of a mile below Bentons mills, Mariposa county, Cali- 

 fornia, Tvhere they were collected by Dr White and others. 



The narrow form o^ Aucella piochii, Gabb, from Riddles, Oregon,t re- 

 sembles this species closely enough in its form to be considered identical, 

 but it has no radial strise. I compared this last with all the Gold belt Au- 

 cellse in turn and independently before arriving at A. erringioni, var. 

 arcuata, and found it agreed with them all to exactly the same degree as 

 arcuata, and, finally, that, like nrcuata, it was easily referred to Mosqnensis 

 by its right valve, and that that species and not pallasi was its nearest 

 foreign representative, in spite of the resemblances of the left valves. 

 The explanation of this fact is that pallasi-like forms, being simply 

 narrow shells, as contrasted with broad ones, may occur in varieties of any 

 species o^Aucella, and the test that such resemblances are not a matter of 

 genetic afiinit}", but of simple morphic equivalence, lies in the character- 

 istics of the least modified part of the shell, the right valve. 



Aucella elongata, n. s. 

 Aucella erringtoni, Meek. Geol. Cal., vol. i, appendix, pi. i, figs. o-5e (not 



figs. 1-4). 

 Loc, Stanislaus river opposite Bostwicks bar. 



This species has an excessively elongated shell, but the outline of um- 

 bonal ridge is apparently straight or only very slightly curved. The 

 shell is heavily striated in all specimens, the striations being generally 

 distributed. The stride being much lighter, and therefore easily oblit- 

 erated by pressure on the extreme anterior and posterior areas, they may 

 be absent upon either or both of these in large specimens. This accounts 

 for the discrepancy^ in the descriptions of Meek and Gabb, one finding 

 them more strongly developed on the anterior and the other on the pos- 

 terior region, but they are apparently, as stated by Meek, usually lighter 

 on the posterior region. It is to be noted, however, that none of the 

 specimens here described have such bare posterior regions as some of 

 those figured by Meek. They are all more similar to his figure 5 than to 

 5 a-e. The hinge line of the right valve rounds out anteriorly more than 

 in Aucella erringtoni, var. arcuata; the oral region is more protuberant. 



* Pinart: Voyage a la c6te Nord-ouest de TAmer., pi. A. 



t Paleontologj' of California, vol. i, pi. 25, fig. 17'^ (not vol. ii, pi. .32). 



