CLARK: NEW BLASTOIDS AND BRACHIOPODS. 367 



■characteristics agree so closely that there can be no doubt that all 

 belong to the same species. 



Description. — Body ovoid, outline rounded, widest just below the 

 middle, and above the base of the ambulacra. Summit flat, base 

 obconical, the opposite sides converging at an angle of about 70°. 

 The radial plates are stout, each fork nearly, if not quite, flat; the 

 suture between adjacent radials slightly depressed. The deltoid 

 plates are large, quadrangular, half as long as the ambulacral areas, 

 and half as wide as long; not quite reaching the summit. The 

 interambulacral areas are furnished with a high, sharply defined rim 

 where they border upon the ambulacra. The ambulacra are about 

 three quarters the length of the body, with sides diverging uniformly 

 to near the summit; median food-groove distinctly depressed. Angu- 

 lar ridges are present on the radials directly below the ambulacra, 

 but they are not prominent. The poral plates are directed upwards. 



Dimensions of two specimens in millimeters. 



Height 20. 20. 



Maximum diameter 16 . 15 . 



Length of ambulacral area 16. 15 . 



Maximum width of deltoid plates 4.5 5. 



Length of deltoid plates 7 . — 



Maximum width of deltoid plates 3.5 — 



Average number of side-plates in 5 mm 15. 15. 



Formation and locality. — All the specimens are from the limestone 

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

 Montana. 



Pentremites altimarginatus differs from Petitremites conoideus (Miss- 

 issippian of Indiana, Illinois, Iowa, Missouri, and Montana) and 

 Pentremites angustus (Chester limestone of Arkansas, Trans. Acad, 

 sci. St. Louis, 1903, 13, p. 53, fig. 14a-b, and Pennsylvanian of Ar- 

 kansas, Bull. Sci. lab. Denison univ., 1915, 18, p. 100, pi. 3, fig. 10- 

 13a) in its more rounded form and in the two latter species the base 

 is truncate and the long, narrow ambulacra extend almost the whole 

 length of the body. Pentremites angustus has larger deltoid plates 

 .and an average of seventeen side-plates in five millimeters. 



Pentremites hurlingtonensis (Burlington limestone of Iowa) has 



