Report or the State Geologist. 603 



pyramidal, becoming irregular in its growth toward the apex. 

 The interior of the pedicle-valve bears a high median septum. 

 extending Longitudinally through the center of the muscular area, 

 which is from one-third to two-thirds the length of the valve; 



near the apex of the umbonal cavity it unites with the dental 

 plates forming a small triangular chamber beneath the deltidumi 



In old shells this cavity is often filled by testaceous secretion. 

 The dental plates do not extend to the bottom of ll| <« rostral 

 cavity except near the apex. 



Brachial valve broad and regularly convex, or sometimes with 

 an undefined median depression, and showing a tendency to auricu- 

 lation at the cardinal extremities. Cardinal area linear; the crural 

 plates are abruptly elevated, and unite to form a strong, erect cardi- 

 nal process, which is distinctly bilobed at the summit; each lobe 

 being slightly notched at the summit and distinctly grooved along 

 the posterior face, these grooves uniting in a wider one at the poste- 

 rior base of the process. Midway in their length these crural plates 

 are marked by a transverse ridge which terminates on their anterior 

 face in points for the attachment of the crura ; on the inside of 

 the dental sockets there is a distinct, sometimes strongly defined 

 ridge, extending along the lateral margin of the broad flabelli- 

 form muscular area, and gradually becoming obsolete. There is 

 also a slight median ridge which becomes obsolete below the 

 middle of the muscular area. Surface marked by strong, sharplv 

 elevated radii, which alternate with finer ones, all being crenu- 

 lated, and the intermediate space cancellated by fine concentric 

 striae. 



Type, Derby a regularis, Waagen. 



Distribution. Carboniferous. 



Meekella, White and St. John. 1868. 

 (Plate 17, Figs. 10-13.) 



Shell robust, biconvex, often subpyramidal. Hinge-line straight 

 and shorter than the greatest width of the shell. On the pedicle- 

 valve the cardinal area is often greatly elevated and distorted ; 

 the delthyrium is covered ; the convex portion of the deltidium 

 being much narrower than the space between the dental ridges. 

 In the interior the teeth are prominent and supported by septi- 

 form dental lamellae which reach the bottom of the umbonal 



155 



