34 PRIMORDIAL PERIOD. 



out; ti-ansverse rugte numerous, some of them inteiTupted, but others 

 extending from the outer borders to the middle of the median furrow, arcli- 

 ing shghtly as they cross the lobes. 



Length of the body in proportion with the width variable ; in some 

 specimens (perhaps broken ones) the length and -nddth being about equal, 

 while in others the length is two or three times as great as the width. The 

 width varies in different specimens from three and a half to more than four 

 centimeters. It is not improbable that this species reached a much greater 

 length than is indicated by any of the specimens in the collection, and that 

 even the longest of these are only fragments. No indications of a stipe 

 arising from the median furrow, such as is seen in C. Linnarrsoni and other 

 species, have been observed. 



This species is larger than C. Linnarrsoni, with which it is associated ; 

 of very different aspect and of different and more variable proportions. It 

 somewhat resembles C. bilohata {Fiicoides hilohatus Vanuxem, Geology of 

 the Third District of New York, page 79) of the Clinton group; but the 

 proportions of the body and of the lobes, respectively, are different. 



Position and locality. — Tonto shale, probably of the Primordial period. 

 Grand Canon of the Colorado River, Mohave County, Arizona. 



ANIMALIA. 



MOLLUSCA. 



MOLLUSCOIDEA. 



Class BRACHIOPODA. 



Order LYOPOMATA. 

 Family DISCINID^. 



Genus ACROTRETA Kutorga, 1848. 



Acrotreta? subsidua White. 



Plate I, fig. 3 a, h, c, and d. 



Acrotreta? subsidua White, 1874, Geog. & Geol. Exp. & Surv. west 100th Merid., Prelim. 



Rep. Invert. Fos.s., 6. 



Shell thin, corneus, discoid, subcircular or somewhat suboval in out- 

 line, the transverse diameter being a trifle greater than the longitudinal ; 



