296 S. WELLER FAUNA OF THE FERN GLEN FORMATION 



12 millimeters is perhaps an average size for the Fern Glen examples, 

 although some of the larger ones grow to be 25 millimeters or more in 

 length. No specific differences can be detected, however, between the 

 larger and smaller individuals associated in this fauna, nor between these 

 and members of the same species elsewhere. The American examples of 

 this shell have usually been considered as either varietally or specifically 

 distinct from the European R. michelinia, but a careful examination of 

 many specimens from various localities in both America and Europe has 

 failed to bring out constant characters of any sort which can be used co 

 distinguish the European from the American examples. 



The shell which Miller has called Orthis dalyana, from Lake valley, 

 New Mexico, is perhaps identical with this Rhipidomella michelinia of 

 the Mississippi valley; it is certainly a member of the same genus, and if 

 not identical is at least a closely allied species. 



SCHIZOPEORIA SW ALLOY I Hall 



Plate 12, figures 6-7 



1848. Orthis resupinata Christy, Letters. on Geology, plate 3, figures 1-2. 

 1858. Orthis swallovi Hall, Geology of Iowa, volume 1, part 2, page 597, plate 



12, figures oa-o. 

 1892. Orthis (Schizophoria) sicallovi Hall & Clarke, Paleontology of New 



York, volume 8, part 1, plate 6, figures 23-24. 

 1894. Orthis swallovi Keyes, Missouri Geological Survey, volume 5, page 63, 



plate 38, figure 5. 



1899. Schizophoria sicallovi Weller, Transactions of the Saint Louis Academy 



of Science, volume 9, page 13, plate 4, figure 7. 



1900. Schizophoria sicallovi Weller, Transactions of the Saint Louis Academy 



of Science, volume 10, page 66, plate 1, figures 11-13. 



Description. — Shell biconvex, resupinate, transversely subelliptical in 

 outline, the hinge line shorter than the greatest width of the shell, the 

 cardinal extremities rounded. Pedicle valve depressed convex, most 

 prominent on the umbo, the surface sloping rather abruptly to the car- 

 dinal margin and more gently toward the lateral and antero-lateral mar- 

 gins, the mesial portion flattened toward the front and sometimes, in the 

 larger individuals, depressed into a more or less conspicuous mesial sinus 

 which is ill defined laterally; beak rather small, moderately incurved; 

 cardinal margin of moderate height, sloping slightly backward from the 

 plane of the valve and gently arched to the point of the beak, the del- 

 thyrium about as wide as high. Brachial valve much more strongly con- 

 vex than the pedicle, most prominent near the middle, the surface slop- 

 ing rather abruptly to all sides, sometimes with an ill defined mesial fold 

 toward the front, the umbo prominent, produced backward beyond the 



