24 MESOZOIC AND CENOZOIC ECHINODERMATA OF THE UNITED STATES. 



Dimensions. — Test : probable diameter about 35 millimeters ; probable height about 25 milli- 

 meters: size of interambulacral plate near ambitus: Width 8 millimeters; height 6 millimeters. 



Description. — The portion of the test studied in the single specimen of this species thus far 

 found is in the form of a cast of the exterior of three interambulacral plates with their adjacent 

 ambulacral plates on either side. The casts afford a very perfect mold of the more important 

 features of these plates, which are apparently below the ambitus, in this respect differing from 

 the specimen of 0. shastensis, which comes apparently from nearer the ambitus. 



The test was probably of medium size. The ambulacral areas are narrow, strongly nexuous, 

 with two rows of small granules between the poriferous avenues. The interambulacral areas are 

 of moderate width, being largely occupied by the tubercles, which are of medium size. The 

 areolas are oval, the boss crenulated and the mamelon perforated. The miliary space is very 

 narrow, very few granules being found on the area. 



Related forms. — Form is shown by its small miliary areas to be quite distinct from 0. 

 shastensis. It is evidently a distinct species. 



Locality. — Two and a half miles west of Round Mountain, Shasta County, Cal. 



Geologic horizon. — "Cedar formation," Upper Triassic. 



Collection. — U. S. National Museum (31189). 



