60 University Geological Survey of Kansas. 



other closely, so that it would be better to leave D. bennetti for the 

 species, should the two be found identical. D. broadheadi may, 

 perhaps, be varietally distinct from D. bennetti, but our mate- 

 rial would hardly seem to indicate it. 



Derbya cymbula. Plate XII, fig. 10. 



Derby a cymbula Hall and Clarke, Pal. N. Y., viii, pt. i, p. 348, pi. xi-b, 

 ff.2, 3, (1892): Rep. N. Y. St. Geol. 1894, p. 348, pi. vi, ff. 1, 2, (1895). 



Original description (in part) : " Shell large ; marginal outline 

 transversely subelliptical. Hinge line straight, its length be- 

 ing about two-thirds the greatest diameter of the shell. On 

 the pedicle valve the cardinal area is high, its base being one- 

 third longer than its sides, and it may be somewhat unsym- 

 metrical from distortion. Its surface is finely striated both 

 longitudinally and transversely, and is divided into an outer 

 and inner portion by two lines diverging from the apex and 

 meeting the hinge line half way between its extremities and 

 the edges of the deltidium. Deltidium broad at the base, rap- 

 idly narrowing for one-third its length, thence tapering more 

 gradually to the apex ; its surface is marked by a well-defined 

 median groove for its entire extent. The surface of the valve 

 is elevated in the umbonal region, and slopes irregularly to a 

 low depression over the pallial region and about the margins. 

 The brachial valve is broadly concave at the umbo, but rapidly 

 becomes regularly convex, the greatest convexity being in the 

 middle of the valve, whence it slopes almost equally in all di- 

 rections. There is no tendency to irregular growth in this 

 valve. Surface covered with numerous fine, sometimes irregu- 

 lar strise, increasing by implanation. Over the umbonal and 

 pallial regions these stride are of about equal size, but above 

 the margin the tendency to fasciculate arrangement is more 

 apparent." The mesial septum of the pedicle valve very high, 

 attach to the teeth, which extend to the top of the deltidium 

 for about a third of its height, and extending about a third of 

 the distance to the front of the shell, highest at the anterior 

 end. Teeth of the brachial valve narrow, extending well into 

 the opposite valve, where they curve inward, locking around 



