Report of the State Geologist 573 



Subgenus Schizotreta, Kutorga. L848. 



(Plate."). Pigg, 1-4.) 



Shells as in Orbiculoidea but having the relative convexity of 

 the valves reversed, the pediole-valve being the more convex. 

 Type, Schizotreta eUvptica^ Kutorga. (Lower and upper 



Silurian, i 



Subgenus Rcemerella, Hall. 1892. 

 (Plate 5, Figs. 9, 10.) 



Brachial valve convex ; pedicle-valve concave. 

 Type, Rcemerella grandis^ Vannxem (sp.) (Hamilton group = 

 Middle Devonian.) 



Subgenus CEhlertella, Hall. 1892. 

 (Plate 5, Figs. 5-8.) 



Contour as in Orbiculoidea. Attachment aided by the pro- 

 jecting margin of the brachial valve. Pedicle-area conspicuously 

 elevated; pedicle-passage an open fissure extending to the margin 

 of the valve. Floor of the pedicle-passage (listrium) tripartite. 

 Muscular scars not observed. External surface covered with 

 concentric striae. 



Type, CEhlertella plea rites, Meek (sp.) (Waverly sandstone = 

 lower Carboniferous.) 



Subgenus Lindstrcemella, Hall. 1892. 

 (Plate 5, Figs. 18, 19.) 



Shells with outline, contour and pedicle-characters as in Orbi- 

 itl'Udea. Brachial valve with a faint median septum and two 

 strong approximating ridges or muscular fulcra, beginning just 

 behind a transverse line passing through the apex and rapidly 

 converging to meet the median septum. Anterior adductor scars 

 lying between these ridges and the median septum : a circular 

 muscular scar at the posterior extremity of each ridge. 



In the pedicle- valve, the foramen has essentially the structure 

 of Orbiculoidea, but its distal margins, though lying in close 

 juxtaposition, appear not to have been united. 



Type, Lindstrwmella aspidium, Hall. (Hamilton group = 

 middle Devonian). 



125 



