270 PALEONTOLOGY, 



by European authors. The specimens from the lowest horizon are gen- 

 erahy more extended on the hinge-hne, and sometimes quite elongated; 

 while those from the lower beds are seldom much longer than the width of 

 the shell below, and in some stages of growth appear to have been short 

 and rounded at the cardinal extremities. There is also a perceptible 

 difference in the character of the striae; those from the higher beds being 

 more finely marked, more angular, and more distinctly fasciculate than the 

 others. 



Formation and locality. — In limestone of the Lower Carboniferous age, 

 near the base of the section, at Dry Canon, and in the higher beds at Snow- 

 storm Hill, Oquirrh Mountains, Utah. Collected by J. E. Clayton. 



Spiripera setigera. 



Plate 5, tigs. 17-18. 

 Spiri/er setigeras Hall, Geo!. Eept. Iowa, vol. 1, pt. 2, p. 705, pi. 27, fig. 4. 



Shell rather below the medium size, transversely oval or elliptical, 

 with ventricose valves, and a short, scarcely defined hinge-line and rounded 

 extremities. Ventral valve more ventricose than the dorsal, most strongly 

 arcuate in the upper part; beak small, somewhat pointed and strongly or 

 closely incurved; area small, the margins not distinctly defined, but rounded 

 almost imperceptibly into the general curvature of the valve. Center of 

 the valve marked by a moderately distinct, but narrow mesial depression, 

 traceable from beak to base. Dorsal valve rather gently and evenly convex, 

 the center elevated in a narrow, not distinctly defined, rounded elevation 

 corresponding to the depression of the opposite valve. 



Surface of the shell marked by numerous, rather closely- arranged 

 concentric varices, marking stages of growth at irregular distances, and also 

 by fine, closely-arranged, setose, radiating lines, most distinct just below 

 each concentric line, but becoming indistinct before reaching the next one 

 below. These lines on the natural surface have been elevated and rounded, 

 forming spines at the concentric ridges, but on the exfoliated surface have 

 the appearance of interrupted radiating lines, scarcely raised on the surface 

 of the shell. 



The specimen figured is somewhat imperfect and much distorted by 



