PLATE XXVIII— Continued. 



Cyrtina triquetea, Hall. 



Figs. 34, 35. Views of an average specimen ; showing curxature of the umbo and height of the rardinal 

 area. X 2. 



Hamilton group. Ruck Island, lUhiois. 



Cyrtina lachuv!mo.sa, !S[). iiov. 



Figs. 36, 37. View.s of an average example ; showing the I'egular, slightly incurved cardinal area, and the 



sparsely pustulose exterior. X 2. 

 Fig. 47. An enlargement of the exterior ; showing the large pustules of various sizes. X 5. 

 Waverly group. Richfield, Ohio. 



Cyrtina acutirostris, Sluiinard. 



Fig. 38. An exterior of a brachial valve, unusually extended on the hinge. X 2. 



Figs. 39-42. Views of a normal example ; showing the high, incurved cardinal area in figures 39 and 40, 

 and the strongly lamellose or squamous exterior of the valves in figures 41 and 42. X 2. 



Fig. 44. A portion of a transverse section of the umbonal region of a specimen In which the deltidial cov- 

 ing is lacking ; showing the cavity or tidie formed at the union of the dental lamellse with the 

 median septum. X 3. 



Fig. ,54. The deltidial portion of the pedicle-valve, in which but a part of the deltidial covering is retaine<l ; 

 showing the edge of the median septum penetrating the deltiduU cavity. X 3. 

 Choteau limestone. Pike county, Missouri. 



