418 PROCEEDINGS OF THE NATIONAL MUSEUM. 



sharp flexure line; stria' of growth cross the area parallel witli its base 

 and arch over the cast of the pedicle furrow. The area of the dorsal 

 valve is relatively short, arching forward slightly at the median portion. 

 The only interior markings observed are seen in the casts of the ven- 

 tral valve, where the visceral area and a portion of the main vascular 

 sinuses are imperfectly preserved. 



Observations. — This species strongly recalls in external form 0. (L.) 

 acutanffulus. The material is too imperfect to identify it with the lat- 

 ter. It occurs at a considerable lower geologic horizon, and what is 

 preserved of the interior markings of the ventral valve indicates a 

 considerable difference in the position of the visceral area. 



Formation and locality. — Middle Cambrian, Rome formation, a mile 

 east of Postoak Springs, Roane County, Tennessee. 



Type.— ISO. !i7341i, U.S.N.M. 



OBOLUS (LINGULELLA) WILLISI, new species. 



General form broadly ovate, with the ventral valve obtusely acumi- 

 nate, and the dorsal valve almost transversely ovate, the posterior 

 margin being very broadly obtuse; convexity moderate in the speci- 

 mens preserved in the calcareous sandstone. Surface of shell marked 

 by concentric lines and stria' of growth, and very line irregular stri;e 

 between them; a few specimens show very narrow, slightly irregular, 

 interrupted radiating ridges or undulations; the inner surface of the 

 shell was more or less strongly pitted or punctate; this character varies 

 greatly in casts from the same layer of shale. The shell, as preserved 

 in the argillaceous shale is relatively thin. 



The largest ventral valve, which is shortened slightly by distortion, 

 has a length of 10 mm. ; width, 8.5 mm. A well-preserved dorsal valve 

 8.5 mm. in length has the same width, while another associated dorsal 

 valve 6.5 mm. in length has a width of 7 mm. 



As shown in the casts, the area of the ventral valve is rather long, 

 and is divided midway by a strong cast of the pedicle furrow, and 

 again midway between the pedicle furrow and the lateral margins by 

 a well-defined tlexure line; fine striic of growth cross the area parallel 

 with its base. The area of the dorsal valve is relatively short, and 

 extends far out onto the cardinal slopes; it is marked by clearly 

 defined, but not strong, tlexure lines. Casts of the interior of the 

 ventral valve show traces of the visceral cavity and the main vascular 

 sinuses. In the dorsal valve a narrow median septum is all that is 

 shown, with the exception of faint indications of the central and inte- 

 rior lateral scars and what appears to be the transmedian scar. 



Observations. — This species was at first compared with 0. {L.) lamhorni. 

 Like that, it has a strongly pitted or punctate inner surface, and the 

 ventral valve has the same general outline; the dorsal valve, however, 

 is much more transverse and obtuse, and the central muscle scars in 

 the dorsal valve appear to be somewhat differently located. 



