Beedk.] Carboniferous Invertebrates, 59 



Derbya bennetti. Plate VIII, figs. 8-8c. 



Derbya bennetti Hall and Clarke, Pal. N. V., vm, pt. i, pp. 2(33, 348, pi. 

 xi-a, ff. 34-39. < 1892): Ann. Rep. St. Geol. N. Y. 1894, p. 347, pi. v, ff. 3-8, 



(1895). 



Original description : " Shell subtrihedral in general aspect, 

 quite irregular in its growth. Hinge line short, its extremities 

 on both valves being auriculate. Pedicle valve much more 

 irregular in growth, sometimes retaining the scar of attachment 

 at its apex. Cardinal area usually high, narrow, erect or 

 slightly incurved, and frequently distorted ; delthyrium curved. 

 General surface of the valve depressed, convex in the middle, 

 sometimes rapidly sloping in all directions, at others concave in 

 the umbonal region ; as a rule \ery unsymmetrical. The 

 brachial valve is deep, more regularly convex, and has a full 

 rounded umbo and a conspicuous median sinus. On the in- 

 terior the pedicle valve bears an extremely high median septum 

 which is united with the dental ridges near the apex. The 

 cardinal process is high, erect and deeply bilobed, each of its 

 apophyses being strongly grooved on its posterior face. Other 

 internal characters unknown. The surface of both valves is 

 covered by fine, elevated, thread-like stria 4 , increasing very 

 slowly by intercalation. The edges of these striae bear numer- 

 ous minute asperities which may be due to the crossing of fine 

 concentric lines. Irregular lines and wrinkles of growth are 

 abundant near the margins." 



Range and distribution : Upper Coal Measures ; Lecompton. 

 Mr. Bennett informs me that the type specimen which was sent 

 Professor Hall, and upon which the description was based, was 

 from near St. Joseph, Mo., instead of Kansas City. 



Hall and Clarke described another species from Kansas City 

 (D. broadheadi) with very much the same characteristics as the 

 species given above. Among the specimens before me I am 

 unable to separate the two satisfactorily, and believe that they 

 will have to be united under a single species. D. broadheadi , in 

 some of its forms, approaches 1). affiuis so closely, being so 

 slightly bilobate, that the two forms seem to merge completely, 

 as well as the characters of the beaks, which also approach each 



