3(5 PRIMOIIDIAL PERIOD. 



llie external surface. The latter is supposed to be lamellose or otherwise 

 so roufflienecl as to have caused it to adhere to the shale, while the smooth 

 interior surface has readily sejDarated in the plane of fission. 



Tliis shell is not only specifically different from any other known to 

 me; but in its want of a well-defined area and in its discoid, instead of 

 pyramidal, form, it difl'ers perhaps generically from those Discinidce that 

 are usually referred to the genus Acrotreta. I have refen-ed it to that genus 

 provisionally, because the combination of its characters renders its reference 

 to any other established genus known to me equally inconsistent, and 

 because the specimens are not complete enough to base a new generic 

 description upon which the species may or may not possess. 



Length of the largest specimen in the collection, six millimeters ; width 

 of the same, seven millimeters. 



Position and locality. — Strata of the Primoi"dial period, probably of the 

 epoch of the Potsdam sandstone, Antelope Spring, House range, Utah. 



Genus TEEMATIS Sharpc, 1817. 

 Trematis pannulus White. 



Trematis pannulus White, 1874, Geog. & Geol. Exp. & Surv. we.st 100th Merid., 

 Prelim. Eep. Invert. Foss., 6. 



Associated with Olcncllus Gilherti Meek, a single imperfect specimen 

 of Trematis has been discovered, which, although consisting only of a 

 single valve, possesses such characteristic surface-markings as to indicate 

 its specific separation from all other known forms of the genus. 



The diameter of the specimen is about three milhmeters ; outline 

 apparently subcircular or a little broader than long-; apex moderately 

 prominent and situated near the posterior margin. Sm-face marked by a very 

 fine net-work of oblique raised lines, dividing it up into minute, four-sided, 

 pore-like pits, which cause it to resemble, under the lens, the texture of 

 finely-woven cloth. 



In the character of its surface-markings, this species is nearly related 

 to T. pimctata Sowerby, sp., as figured by Davidson in his Monograph of 

 B)-itish Fossil Brachiopoda, part vii, No. 1. That species, however, reaches 



