CRUSTACEA— OSTRACODA. 351 



more typical forms resembles the figure 8. Genotype 0. octo- 

 formis Jones. Ten or more species. Ordovicic-Devonic. 



47. 0. curta Ulrich. (Fig. 1666, /, /'.) Siluric. 

 Valves deep; sides nearly rectangular; surface with thick, oval, 



ring-like elevation. 



Rochester shale of New York. 



48. 0. clavigera Ulrich. (Fig. 1666, w,m',n.) Devonic. 

 Elongate; surface with club-shaped ridge. 



Bryozoan bed (Onondaga) of Falls of Ohio. 



49. 0. stigmata Ulrich. (Fig. 1666, 0,0'.) Devonic. 

 Outer ring-like ridge enclosing several others of less regular 



outline. 



With the preceding. 



XXIV. Placentula Jones and Holl. 

 Probably related to Bollia, but differing in having the " loop " 

 generally in front of the center and close to the dorsal margin. 

 As a rule a rim-like ridge forms the outer border of the valves. 

 Genotype P. excavata J. and H. Five or six species. Ordovicic 

 and Siluric. 



50. P. marginata Ulrich. {¥\g.i666,d,d'.) Ordovicic. 

 With small, indistinct subcentral loop, and strong marginal 



ridge, almost spinous at dorsal margin. 

 Cincinnatian of Cincinnati region. 



XXV. Bollia Jones and Holl. 

 Distinguished by a centrally situated loop-like or horse-shoe- 

 shaped ridge, the upper extremities of which are often bulbous ; a 

 more or less complete marginal ridge may be present or wanting. 

 Genotype B. uniflexa J. and H. Twenty-five species. Ordovicic- 

 Carbonic. 



51. B. unguloidea Ulrich. (Fig. 1658, ,r,.r'.) Ordovicic. 

 Marginal rim thick; inner U-shaped ridge close to it; arms of 



unequal length, scarcely expanded dorsally. 

 Trenton of Minnesota. 



52. B. pumila Ulrich. (Fig. 1660,^, y.) Ordovicic. 

 Inner U-shaped ridge thin, distant from marginal ridge, and 



with swollen ends. 



Upper Cincinnatian of Ohio, Indiana, etc. 



