364 bulletin: museum of comparative zoology. 



Dimensions of the three specimejis adjusted to a height of 20 millimeters. 



Height 20. 



Maximum diameter 13 .8 



Length of ambulacral area 13.8 



Maximum width of ambulacral plates . . 5.8 



Length of deltoid plates 3.8 



Maximum width of deltoid plates 2.3 



Formation and locality. — I collected this species from the limestone 

 member of the Quadrant formation at Old Baldy, near Virginia City, 

 Montana, and at Squaw Creek off the West Gallatin River. 



Pentreinites symmetricus (Kaskaskia limestone of Kentucky) most 

 nearly resembles Pentremites saxiomontanvs, but its interambulacral 

 areas are concave; in Pentremites saxiomo7itanus they are flat. The 

 base of the latter species is not so stout, and the summit has a smaller 

 area. The lesser prominence of the median ridges on the radials in 

 Pentremites symmetricus, too, is a marked difference. The angle 

 made by the sutures between the deltoid and radial plates in Pentre- 

 mites symmetricus is obtuse, in Pentremites saxiomontanu^ very acute. 



Pentremites pyriformis (Mississippian of Alabama, Tennessee, Ken- 

 tucky, Illinois, Indiana, Missouri) is thickest in the middle and 

 rounded in outline, while Pentremites sa.viomontanus is decidedly 

 angular. The deltoids are smaller in Pentremites pyriformis, and the 

 basal portion is more elongate. 



One of the specimens, as etched from the matrix, showed a very 

 slender column, which would help to distinguish it from Pentremites 

 gemmiformis (Kaskaskia of Illinois). The latter species possesses a 

 rim around the ambulacral area and sixteen side-plates in five milli- 

 meters. 



In Pentremites elegans (Millstone grit of Kentucky, Maxville lime- 

 stone of Ohio) the deltoids are small, occupying but one sixth the 

 length of the body. The ambulacra are, relatively, considerably 

 shorter than those of Pentremites' saxiomontanus, and in outline too 

 much rounded below. Moreover in Pentremites elegans there are 

 twenty-one side-plates in five millimeters. 



