238 



which slight prominence the valves are abruptly and obliquely com- 

 pressed as well as somewhat excavated. Length nearly twice as great 

 as the height : lateral outline subelliptical : anterior end truncated 

 inwardly and obliquely under the beaks : posterior end narrowly 

 rounded : superior border regularly, convexly and very broadly arched : 

 ventral margin nearly straight or slightly concave in advance of the 

 middle and as slightly convex behind, forming an obtuse angle with the 

 anterior margin and a rounded junction with the posterior : beaks 

 anterior, terminal and overhanging, approximated and curved down- 

 wards as well as inwards. 



Surface marked by a few faint concentric lines of growth. In speci- 

 mens which have the exceedingly thin outer layer of the test exfoliated, 

 however, which is often the case, the outer surface of the inner layer of 

 the shell, when examined under a lens is seen to be marked with 

 crowded radiating striae which are too minute to be visible to the naked 

 eye. 



Length of an average specimen seventeen millimetres : greatest 

 height of the same, nine mm. : maximum thickness, nine mm. 



East end of Maud Island : four specimens with both valves. 



OXYTOMA MUCRONATA, Meek. 



Plate 31, fig. 9. 



Pteria ( Oxytoma) Munateri, Meek and Hayden. — 1864. PalEeontology of the 



Upper Missouri, p. 80, 

 wood cuts, figs. A, B. 



Aiictda {Oxytoma) mucronata, y^hltfield. — 1876. Palaeontology of the 



Black Hills of Dakota, 

 p. 357. pi. 4, figs. 1,2. 



East side of Alliford Bay, one left valve, which is figured. The same 

 species also occurs in the Lower Sandstones, oi" sub-division E, of the 

 South side of Maud Island. 



The tyjDes of 0. inucronata from the Black Hills of Dakota were 

 originally regarded by Meek, though with much doubt, as possibly 

 identical with the Avicula Munsteri of Brown and Goldfuss. Eichwald, 

 however, in the Lethaea Rossica,* says that the sculpture of the left 

 valve of A. Munsteri consists of from sixteen to eighteen radiating ribs, 

 with intervals furnished with one to three radiating striae, whereas in 

 large left valves of 0. mucronata from Maud Island the primary radia- 



* Volume 2, page 504. 



