602 Forty-fifth Report on tee State Museum. 



the umbonal cavity of the opposite valve ; it is supported by a 

 short, median septum, and laterally by strong crural plates which 

 extend for a short distance along the margins of the muscular 

 impressions ; these are usually quite faint and undefined, occupy- 

 ing a much smaller area than in the pedicle-valve, and leaving 

 arborescent markings as in some species of Stropheodonta. On 

 the interior of both val'ves the margin is sharply pectinated, or 

 crenulated, the crenulations on the brachial valve extending to 

 the base of the cardinal process, and in the pedicle-valve extend- 

 ing for a considerable distance on the cardinal area. Surface of 

 both valves covered with fine sharp radiating striae. Shell sub- 

 stance punctate. 



Type, Hipparionyx proximus, Yanuxem. 



Distribution. Lower Devonian (Oriskany sandstone; but a 

 single species is known). 



Kayserella, Hall. 1892. 



Shell subsemicircular ; hinge-line nearly equaling in length the 

 greatest width of the shell; convexity normal. Pedicle- valve 

 more convex than the brachial ; area moderately high ; delthyrium 

 covered by a convex plate. Brachial valve slightly convex ; area 

 narrow ; chilidium well developed. In the interior, a large triangu- 

 lar median septum extends almost to the bottom of the opposite 



& 



Figs. 275, 276, 277.— Kayserella lepida. After Kayser. 



valve. Surface with a slight median ridge and furrow on pedicle 

 and brachial valves respectively ; covered with sharp plications, 

 increasing by intercalation near the margin. Shell substance 

 punctate. 



Type, Kayserella lepida, Schnur (sp.). 



Distribution. Middle Devonian (Eifel; but a single spoeies is 



known). 



Derbya, Waagen. 1884. 

 (Plate 17, Figs. 1-9.) 



General character of the shell as in Orthothetbs. The externa] 

 form is more variable, being usually plano-convex, sometimes 

 concavo-convex, the pedicle-valve often t lie more elevated and sub- 



154 



