OSTEACODA. 



669 



BoUia subsquata.] 



sometimes thicker at one end than the other, and the depressions or sulci between 

 the ridges relatively narrow. These species pass over into, and the section ought to 

 include, some of the so-called quadri-jugate Beyrichice* Their relations to Tetradella 

 will be considered in the remarks under that genus. 



BOLLIA SUB^QUATA, n. Sp. 

 PLATE XLVI, FIGS. 28-29. 



Size.— Length, 0.64 mm.; bight, 0.'50 mm.; thickness 0.22 mm. 

 Length, 0.59 mmt; hight, 0.42 mm.; thiclcness, 0.20 mm. 

 Length, 0.40 mm.; hight, 0.32 nim. 



Valves compressed, subovate in outline, straight above; length of hinge some- 

 what variable, shortest, apparently, in old examples; dorsal angles more or less 

 distinct; edges of valves thick, forming a sharply-defined, thin marginal ridge; mner 

 ridge thin, rather long, U-shaped, one of the arms with a slight swelling near or at 

 its upper extremity; surface between the ridges flat and without ornament. 



The small valves of this species remind somewhat of Moorea punctata, but as that 

 form has no inner or horseshoe ridge they are distinguished very easily. Tetradella 

 quadrilirata is a larger form and has the inner ridges joined below with the marginal 

 ridge. The nearest allies occur in the Upper Silurian deposits of Europe, but it is 

 distinct from them all. 



Formation and locality .—OtsXeaa, shales (Clitambonites bed), near Cannon Falls, Minnesota. 



BOLLIA UNGULOIDEA, «. Sp. 



Size.- 



PLATE XLVr, FIGS. 23-25. 



-Length 0.6 mm.; hight 0.4 mm.; thickness 0.35 mm. 



Valves subovate, with equal rounded ends, a rather short, straight hinge, and 

 illy defined dorsal angles; marginal ridge thick, rounded, a little wider at one end 

 than at the other; inner ridge strong, one end swollen, the other small and failing 

 to reach the dorsal edge; interspace between the two ridges very narrow. 



* With the exception of several very doubtful species described by Krause, the Icuown species fall into these sections, 

 as follows: 



