ORTHOTETES 77 



that if it be considered advisable to retain the two groups as of generic 

 value the name Orthotetes would be applicable to one and Derbya to the 

 other. This procedure seems to be unwarranted, however, for the reason 

 that the species under consideration, 0. keokuk, is essentially intermedi- 

 ate in character between the two types. In it, the dental plates cannot be 

 said to be septate, but near the apex of the valve they usually do join the 

 median septum in such a manner as to inclose a short triangular chamber, 

 but not a deep one extending from the apex to or nearly to the hinge-line, 

 as in the case of the more typical representatives of the camerate division. 

 In some examples of 0. keokuk the triangular chamber may be scarcely 

 distinguishable at all, and in one specimen observed the dental plate is 

 joined with the median septum on one side but not on the other. 



This species is the largest representative of this type of shell in our 

 Mississippian faunas, and sometimes attains a width of 100 millimeters. 



Horizon. Keokuk limestone. 



Plate VI, Figs. 1-14, 15? 



1860. Orthis kaskaskiensis McChesney, Desc. New Pal. Foss., p. 31. 

 1892. Derbya kaskaskiensis Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, pi. 

 fig. 6. 



Description. Shell above medium size, subelliptical or subcircular in 

 outline, often somewhat distorted, broader than long, the greatest width 

 near the mid-length of the shell, the hinge-line shorter than the greatest 

 width, the cardinal extremities rounded or obtusely angular. The dimen- 

 sions of two nearly perfect specimens are : length of pedicle valve 41 mm. 

 and 28.3 mm., length of brachial valve 37.7 mm. and 25.2 mm., greatest 

 width 49.5 mm. and 34.8 mm., length of hinge-line 35.5 mm. and 28.5 mm., 

 height of cardinal area 9 mm. and 5.6 mm., thickness from deepest part 

 of brachial valve to apex of pedicle valve 19.5 mm. and 14 mm. 



Pedicle valve nearly flat or somewhat concave except in the umbonal 

 region where the beak is frequently rather abruptly elevated, sharply 

 pointed and more or less distorted ; mesial sinus obsolete ; cardinal area 

 nearly flat, sometimes a little concave or convex towards the apex, sloping 

 posteriorly from the hinge-line at an angle of from 100 to 140 degrees, 

 most commonly about 120 degrees, the lateral margins well defined and 

 sharply angular, sloping from the apex of the beak to the cardinal ex- 

 tremities in nearly a straight line or with a slight concavity near the 

 beak and a slight convexity near the cardinal extremities; delthyrium 

 much higher than wide, closed by a rather strongly convex deltidium 

 which is usually longitudinally flattened or slightly sinuate. Internally 

 a strong median septum originates at the beak and extends anteriorly, 

 the dental plates are short, nonseptate, being scarcely more than thick- 



