Geol.— Vol. L] SMITH— COMPARATIVE STRATIGRAPHY. 387 



median ridge. The septa are ceratitic, but both lobes and saddles are 

 denticulated. 



In youth the form is more e volute and robust, and the cross-section of 

 the whorl is nearly circular, with the exception of the impressed zone. The 

 ribs are fine and much straighter than at maturity, and in the earliest stages 

 are present only on the umbilical shoulders. 



The length of the body-chamber is nearly or quite a complete revolution, 

 and this increases the resemblance to Paratropites, but the young stages are 

 not like those of the Tropitidas. 



G. von Arthaber^ has described in the genus Reijlingites 

 a somewhat similar group, that resembles Ceratites in form, 

 but differs in the long body-chamber. The writer, how- 

 ever, is not of the opinion that the length of the body- 

 chamber is of paramount taxonomic value. 



Horizon and locality. Middle Trias, Daonella beds, 

 West Humboldt Range, Nevada, in Star Canyon, Cotton- 

 wood Canyon, and on the divide between Troy Canyon and 

 the south fork of American Canyon. The figured speci- 

 mens were collected by the writer at the last locality, 

 associated with Beyrichites rotelliformis, Acrochor dicer as 

 hyatti, Anolcites whitneyi, A. meeki, and many other char- 

 acteristic Middle Triassic species. 



Genus Trachyceras Laube. 



This genus was named by Laube ^ with " Ceratites'''' aon 

 Muenster as the type. Since that time Mojsisovics has 

 extended and redefined the genus, dividing it into three 

 groups, or subgenera, Trachyceras s. str., Anolcites, and 

 Protrachyceras. 



Subgenus Protrachyceras Mojsisovics. 



1893. Protrachyceras Mojsisovics, Cephalopoden der Hallstiitter Kalke, 



II, p. 618. 

 1896. Protrachyceras Mojsisovics, Beitr. Kennt. Obertriadischen Cephalo- 



poden-Faunen des Himalayas, p. 646. 

 1898. Protrachyceras Tornquist, Zeitschr. Deutsch. Geol. Gesell. Bd. L., 



Heft 4, p. 659. 



iSee Bibliog. i — p. 72. 



^Ueber Ammonites aon und Verwandte Arten, Sitzb. k. Akad. Wiss. Wien, 

 Bd. UX, p. 7. 



