366 bulletin: museum of comparative zoology. 



This species can be distinguished from either Pentremites alti- 

 marginatus or P. saxiomontanus by its rotund form. 



Pentremites cemnus (Mississippian of Alabama and lUinois) is more 

 angular, especially below the ends of the ambulacra. In Pentremites 

 fUrergens the deltoids are more than twice as long as wide, while in 

 Pentremites cercitius, according to the figure they are only one half 

 longer than wide, therefore less mucronate. The ambulacra expand 

 very gradually toward their widest part. 



Pentremiies cherokeus (Mississippian of Alabama, Illinois, and 

 Tennessee) is a larger form whose deltoids project above the oral 

 aperture. The angles at the bases of the ambulacra are too promi- 

 nent to be Pentremites divergens, and the interambulacral areas are 

 deeply concave. Moreover Pentremites cherokeus is considerably 

 thicker than high and the summit (including the deltoids) is two 

 thirds as wide as the maximum thickness of the specimen. 



Pentremites koninckanns (Mississippian of Indiana, Illinois, Iowa, 

 Missouri) has much shorter ambulacra and deltoids. 



Pentremites bradleyi (Carboniferous of Montana) has its ambulacra 

 deeply excavated along the middle. In Pentremites divergens the 

 ambulacra are scarcely grooved at all. The flat base of Pentremites 

 bradleyi is the most distinguishing feature of the species. The latter 

 species has nineteen side-plates in five millimeters, Pentremites di- 

 tergens fifteen to sixteen. 



Pentremites rusticus Hambach (Trans. Acad. sci. St. Louis, 1903, 

 13, p. 54, fig. 15) has a subcylindrical body, with the upper portion of 

 the interambulacral areas strongly elevated above the ambulacrals. 

 The base is flattened or concave. This species Avas described by 

 Hambach from the Chester limestone of Arkansas, and discovered by 

 K. F. Mather, along with Pentremites angustus, in beds of Penn- 

 sylvanian age in Arkansas (Bull. Sci. lab. Denison univ., 1915, 18, 

 p. 101, pi. 3, fig. 3-6a). 



This species takes its name from the widely diverging margins of 

 the ambulacral areas. 



Pentremites altimarginatus, sp. nov. 



Plate 1, fig. 11-13. 



This species, which includes the largest forms so far found at Old 

 liald^>', is represented l)y four specimens, all of whose observable 



