6 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



oval, a little arcuate, the convex side being toward the front, located about 

 one-fourth the length of the fossil from the anterior side; vent small, and a 

 little higher than wide ; apical disk not more than one-third the length in 

 advance of the posterior end ; genital pores rather larger ; dorsal ambulacra, 

 excepting the odd one, distinctly petaloid, and very unequal ; anterior or odd 

 one long, lanceolate, and extending nearly to the margin, located in a rather 

 deep, rounded groove, which passes over the front to the mouth, giving a 

 faintly emarginate outline to the anterior end, provided with about thirty-three 

 pairs of pores in each series ; anterior lateral ambulacra slightly flexuous, a 

 little broader than the odd one, and not quite so long, placed in rather distinct 

 grooves, and having about thirty-five or thirty-six pairs of pores in each 

 series ; posterior ambulacra very small, not more than about one-third as long 

 as the anterior laterals, oval in form, a little depressed, but not deeply exca- 

 vated, and having some eighteen pairs of pores in each series ; interambulacral 

 spaces rather prominent, the anterior two being a little pinched up ; surface 

 unknown. 



Length, 1.21 inches; height, 0.81 inch; breadth, 1.20 inches. 



Not having seen specimens showing the surface-markings of this species, 

 it is with some doubt that it has been referred to the genus Hemiaster, since it 

 is impossible to determine from the specimens whether or not it possessed the 

 peculiar fasciole characteristic of that genus. The posterior position of the 

 apical disk, and the close proximity of the mouth to the front, as well as the 

 great disparity of size between the posterior and anterior lateral ambulacra, 

 are characters in which it differs from forms usually referred to the genus 

 Toxaster. In some respects it resembles species of the genus Schizaster, 

 but it does not decline so distinctly toward the front above, and is not so 

 angular behind, as is common in that genus. When specimens are obtained 

 showing all its characters, it is possible that there may be found sufficient 

 differences between it and the groups already defined to entitle it to rank 

 as the type of a new section. 



It is a little remarkable, that, in all the collections hitherto obtained from 

 the Cretaceous rocks of the Upper Missouri, this is the only species of 

 Echinoidea yet found, and it is so rare that but two specimens, and a frag- 

 ment of another from the same locality, have been met with. 



The specific name of this interesting fossil was given in honor of Capt. 

 (now General) A. A. Humphreys, of the U. 8. Topographical Engineers. 



