404 A. HYATT — TRIAS AND JURA IN THE WESTERN STATES. 



This is the extreme form, and the shell is in some cases apparently 

 smooth and in others marked by longitudinal stride. Its nearest affine 

 is the Aucella mosqiiensis, Keyserling, as figured and described by Lahusen 

 from the lower Volgian in the Upper Jura of Russia. Aucella pallasi, 

 Keyserling, is sometimes slightl}^ striated and very like, this species in 

 general aspect of the left valve, but the characteristic of the hinge line 

 of the right valve enables one to separate them easily from each other. 

 The explanation is simply that the range of morphic modification in the 

 whole group of the genus, and likely to occur in every species, is from 

 Avicula-like shells with circular outlines to forms with narrow arcuate 

 left valves ; but while two forms may be similar in the shape of the left 

 valve, they may entirely disagree in the shape of the hinge lines of the 

 less modified right valves. This is not uncommon in highly variable 

 asymmetrical Pelecypoda, and in all of these the characteristics of the 

 hinge are admitted to be determinative of the species. 



Comparison of Aucellse. — Aucella conceiitrica, Fischer (not Keyserling), 

 collected at Amackshack bay, Alaska, may be the same as this species. 

 The narrow smooth form of Aucella piochi, Gabb, from Riddles, Oregon, 

 sent me by Mr Diller as coming from the Knoxville group, appears to be 

 very similar to this species. This last is sufficiently distinct from the 

 stout plicated form of Aucella piochi of the Shasta group to be separable 

 on biologic grounds, and it is probably stratigraphically distinct, the 

 mass of the former, occurring below the stouter form of the latter, as has 

 been shown by Messrs Stanton and Diller. The history of these two 

 forms accords exactly with the geologic conformability and unbroken 

 succession of the strata of Knoxville and Shasta groups, as stated by 

 Mr Diller. After a close examination of the large collection in the 

 National Museum I came to the conclusion that, as a rule, the narrow 

 form of Aucella piochi occurred dissociated from the broad plicated form 

 described by the same name, and accordingly furnished Mr Diller with 

 a list of localities to test in the field the question of superposition. 



The following is a part of his field report for 1893 subsequently sent to 

 Mr C. D. Walcott,* which he kindly permits me to publish : 



"After studying the collection of Aucellee at the National Museum last year, 

 Professor Hyatt expressed the opinion that there were apparently two types of 

 Aucella, one Cretaceous and the other Jurassic. Both, however, were from the 

 Knoxville beds. He gave me a list of localities at which the stout or Cretaceous 

 forms occur, and also the localities at which the slender or Jurassic form occurs, 

 and upon examination I found that all the Cretacic forms come from beds over- 

 lying the Jurassic forms. The investigations made this year by Mr Stanton and 

 myself point in the same direction, but I am not sure that the two forms are not 

 intermingled. Mr Stanton made collections of Aucellae at a number of places, and 



* Chief geologist, U. S. Geological Survey. 



