310 SYSTEMATIC PALEONTOLOGY 



" These lines turn backward in crossing the middle of the shell and 

 then forward again on either side. Along the center of the shell runs a 

 narrow carina or slit band which is open for a short distance on the last 

 whorl. The lip shoAvs a broad, deep notch on the outer edge, and at the 

 base of this notch is a further slit. On most specimens this carina is a 

 flat or depressed band, but on a few, especially on young specimens and 

 on the outer whorl of adults, the carina is. elevated." Raymond, 1908. 



This well-known Chazyan fossil is easily recognized by the shape of the 

 shell with a slit band, and especially by its characteristic reticulate surface 

 markings. 



Occurrence. STONES RIVER LIMESTONE (Middle division). Examples 

 have been collected at the old quarry at Chambersburg, Pennsylvania, 

 and south along this band of outcrop into Maryland. Chazyan of the 

 Lake Champlain area and Tennessee. 



Collections. Maryland Geological Survey, TJ. S. National Museum. 



Genus TETRANOTA Ulrich and Scofleld 

 TETRANOTA OBSOLETA Ulrich 



Plate LV, Figs. 22-24 



Tetranota obsoleta Ulrich and Scofield, 1897, Geol. Minnesota, vol. iii, pt. 2, 

 p. 880, pi. Ixv, figs. 19-23. 



Description. This species differs from other members of the genus, 

 and especially from the genotype Tetranota bidorsata, in the fact that the 

 revolving ridges are much less developed, particularly on the last volution. 

 The central lateral pair is quite obsolete except on the inner volutions 

 and even the central pair merely maintains the same strength relatively 

 that it held in earlier stages. Interior casts of the mature shell exhibit 

 a broad, comparatively low, and more or less distinctly grooved central 

 ridge, beyond which the surface is first shallowly excavated and then 

 gently convex to the lateral boundaries of the volutions which again are 

 not angular but rounded. The exterior of the shell appears the same 

 except that the ridges bordering the slit band seem thinner and sharper. 



