366 bulletin: museum of comparative zoology. 



This species can be distinguished from either Pentremites alti- 

 marginatus or P. s axiom ontanus by its rotund form. 



Pentremites cervinus (Mississippian of Alabama and IlHnois) is more 

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

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

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

 longer than wide, therefore less mucronate. The ambulacra expand 

 very gradually toward their widest part. 



Pentremites cherokens (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 koninckanus (Mississippian of Indiana, Illinois, Iowa, 

 Missouri) has much shorter ambulacra and deltoids. 



Pentremites hradleiji (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- 

 mrgens fifteen to sixteen. 



Peyitrcmites rusticus Hambach (Trans. x\cad. sci. St. Louis, 1903, 

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

 the interambulacral areas strongly elevated above th« ambulacrals. 

 The base is flattened or concave. This species, was 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 

 Baldy, is represented by four specimens, all of whose observable 



