COMPARISON OF AUCELL.E. 405 



will be able, when the collections arrive and he studies them, to satisfactorily solve 

 the problem. 



"There is one point, however, concerning which there is no doubt, namely, that 

 both forms occur in the Knoxville beds near together. They are in the same series 

 of sediments laid down without any interruption, and if one is Jurassic and the 

 other Cretaceous, then there is no break between the two, and the Knoxville beds 

 belong to both systems. 



' ' The relation of the Knoxville and Mariposa beds is not yet satisfactorily de- 

 termined, and I have no evidence concerning their relation. The fact, however, 

 that there is no break in the Shasta-Chico series makes it more probable that the 

 Knoxville and Mariposa beds are unconformable." 



The opinion which Mr Diller quotes that the smooth, narrow form of 

 Aticella piochi is probably Jurassic was founded upon the supposition, 

 which then seemed to me reasonable, that it was identical with the stri- 

 ated, narrow form described in this paper as Aucella erringtoni var. arcuata. 

 The late explorations of Mr Stanton and Mr Diller have, however, dem- 

 onstrated, what the collections of the National Museum had alread}^ led 

 me to suspect, that these two were distinct. Not only are nearly all the 

 fossils of the former in the collections of the National Museum smooth, 

 but I am informed that out of the hundreds examined in the field b}^ 

 these two gentlemen and brought home by them only very few have yet 

 been found with striations like those of the Gold belt fossils. Any one 

 familiar with the slight differences that characterize species of this genus 

 will at once see, when the specimens are placed side by side, that not 

 only are the Knoxville forms of Aucella piochi deficient in striations, but 

 they have not the broad posterior wing of the Aucella erringtoni var. arcu- 

 ata and the beaks are more prominent, extending further beyond the 

 hinge line, and proportionately narrower. 



Even if it should be admitted that these two sj^ecies were connected by 

 intermediate forms, this fact would only show that the migration of the 

 Aucella^ took place from the more southern basin into the more northern 

 one, and give a solid biologic argument for the conclusion that the Gold 

 belt slates are older than the Knoxville. The connecting forms are con- 

 fined to one variety of Aucella erringtoni var. arcuata having striations, 

 and to one variety of extreme rarit}' in the Knoxville, Aucella piochi of 

 the slender variety, having striations. The remaining varieties and spe- 

 cies are distinct. The Aucella piochi has its own peculiar series of forms, 

 which, as will be shown by Mr Stanton, pass from the slender form in 

 the lower Knoxville into the stouter constricted shells of the upper 

 Knoxville, and are there associated with Cretacic species of Ammonitina\ 

 The Aucella) of the Mariposa region have their own peculiar series of 

 broad and rounded forms having affinities for the slender Aucella erring- 

 toni var. arcuata, but they evolve along different morphic lines from 



