DESOEtPTIONS OF SPECIES. 479 



the Colorado form is of the same general tj'pe. It is, however, without much doubt 

 a distinct species, but since only a single specimen has come to hand 1 do not feel 

 justitied in giving it a new name. The length is about 1.5 mm., thus making it larger 

 than the Mississippian species, and it is also more transverse. The posterior extremitv 

 is probably no-t as pointed as the older specimen figured by Ulrich (fig. 6), and much 

 less so than the younger one (fig. T). The upper and lower margins are nearly 

 rectilinear and parallel, though thev gradually converge toward the anterior margin, 

 which is rather regularly and strongly rounded. 



This species occurs associated with several osti'acode forms at Empire Hill in 

 the Leadville district (station 2265), a localitj^ which, with larger collections, would 

 probably furnish an extensive fauna of this group. In addition to the types 

 mentioned here under a separate heading, still another very minute form is found 

 whose relations I have not been able to make out. 



Locality and horizon. — Leadville district (station 2266). 



BP:YRICHIA McCoy, 1.S44. 

 Beykichia sp. 



This form is extremely aljundant at Glenwood Springs (station 2193). The 

 following brief description is offered : 



The width is usually from 1 to l.."> mm., and this dimension is from one and 

 one-half to twice the height. The upper margin is long and straight. The ends are 

 strongly, and the inferior margin gently, rounded. The posterior end of the hinge 

 lin(; is more or less angular. The shape is nearly symmetrical with respect to a 

 \eitical axis. Somewhat anterior to the middle, as it appears, the shell is divided 

 by a deep, strong sulcus, which is vertical to the hinge and has a length about 

 one-third as great as th(> height of the shell. Not far anterior to this another fainter 

 groove can usually lie detected. The lower ends of these two depressions are 

 sometimes .seen to be deflected and to unite, thus inclosing a small median lobe. The 

 surface is delicately and beautifully hachured. 



This species appeai-s to be one of the simple Beyrichian types related to B. arcuata 

 of the Carboniferous rocks of Great Britain. 



A closely related and possibly identical form is found at Empire Hill, in the 

 Leadville district. My material is very scanty, and at the same time somewhat 

 imperfect, so that I have not entirely assured myself as to the relations of these two 

 forms. 



LocnlUy. — Grand River region, Glenwood Springs (station 2193). Leadville 

 district (station 2265). 



