DESCRIPTIONS OF UPPER JURA FOSSILS PELECYPODA. 433 



Aucella aviculxjormis^ 30 

 Loc.,near Reynolds ferry, south bank of Tuolumne river, six miles from 



Copperopolis. 



This has an outline similar to that of some species of the genus Avicula. 

 The young are similar to Aucella orbicidarls, but become more elongated 

 in adults without, however, entirely losing the orbicular form. The 

 posterior wing is large and the posterior outline like that of an Avicula 

 in some specimens ; in others it is similar to Posidonia. The oral region 

 is very protuberant and more rounded than that of var. elongata, but does 

 not project so far anteriorly, especially in the right valve, as does the 

 same region in orbicularis. The hinge region is broad and straight, the 

 valve in outline being similar to the right valve of orbicularis ; but the 

 diameter from the umbo to the border of the dorso-ventral edge is longer 

 in proportion to the antero-posterior or transverse diameter. In fact it 

 is just intermediate betw^een elongata and orbicularis in many characters, 

 but differs from both by its peculiar form and outline. The strise are 

 persistent and about the same as in elongata and orbicularis. There is 

 nothing similar in European faunas, so far as I know. This species has 

 the general aspect of Aucella crassicollis, var. solida, but differs in the de- 

 velopment of the posterior wing and striations. Eichwald's figures of 

 Aucella pcdlasi, figures 3 and 4, and Aucella mosquensis, figures 13 and 14^ 

 (no other figures in this plate), resemble this variety closely, but have 

 smooth' shells and the posterior wing is not developed. The Aucella 

 mosquensis, Whiteaves,t is considered by Lahusen X as very similar to his 

 Aucella volgensis, but he has not been able to identify the two. He also 

 makes a comparison between his Aucella moxquensis and the Cretacic 

 Monotis concentrica of Salta, but is not able to identify them. Aucella mos- 

 quensis is not similar to Aviculaeformis. but Volgensis of Lahusen is cer- 

 tainly a representative form in Europe, although in this, as in the other 

 species of European faunas, the development of the posterior wing and 

 absence of radiating striae are marked differences. 



Var. acuta. 

 Loc, six miles from Copperopolis. 



This may be merely a noteworthy form of aviculseformis, or it may 

 indicate another series between elongata and aviculseformis. I think it is 

 simply an extreme form of aviculxformis, having, however, a subacute 

 angle in the lines of growth on the umbonal ridge and a correspondingly 

 subacute outline at the posterior extremity. 



* Mangischalk et Aleutian Inseln., pi. 17. 



t Mesozoic Fossils, vol. i, pt. i, pi. 10. 



J Lahusen, Russichen Aucellen Mem, Com. Geol. St. Petersburg, viii. No. 1. 



