OBOLID^. 535 



anterior portions of the shell, in this respect resembling the shell of Lingulella acutangula 

 (Eoenier) (PI. XVII, figs. Im, lo). 



Casts of the interior of the ventral valve have a moderately long area divided midway by 

 the cast of a strong pedicle furrow and marked about midway between the pedicle furrow and 

 the lateral margm by a sharp flexure line; striae of growth cross the area parallel with 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 ventral valve where the visceral area and a portion of the main vascular 

 sinuses are imperfectly preserved. 



Observations. — The external form of this species strongly recalls that of Lingulella acutangula 

 (Roemer), but the material is too imperfect to identify it with the latter. It occurs at a consider- 

 ably lower geologic horizon, and what is preserved of the ulterior markings of the ventral valve 

 indicates a considerable difference in the position of the visceral area (PI. XVII, fig. Ic, and 

 PI. XXIII, fig. 2a). 



Formation and locality. — Middle Cambrian : (11) Sandstones and shales of the Rome formation, about 1 mile 

 {1.6 hm.) east of Post Oak Springs [Hayes, 1894, areal geology sheet], Roane County; and (11a) sandstone between First 

 and Armstrong creeks, in the southeast corner of the Maynardville quadrangle (U. S. Geol. Survey), Union County; 

 both in Tennessee. 



Lingulella texana Walcott. 



I, 



Plate XLIX, figures 3, 3a. 



Lingulella texana Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 3, p. 71, PI. VIII, fig. 5. (Characterized as 

 below as a new species. Fig. 5 is copied in this monograph, PL XLIX, fig. 3.) 



This is a small but distinctly marked species, represented by two dorsal valves occurring 

 in the Upper Cambrian limestones of central Texas. The dorsal valves are oval, and quite 

 strongly convex. The shell appears to have been rather thick, and the outer surface is marked 

 by strong, radiating strise, a feature which is characteristic of the species. They are crossed 

 by fine, concentric striae and lines of growth. The position of the muscle scars and the size and 

 character of the area are shown by Plate XLIX, figure 3a. 



Formation and locality. — Upper Cambrian: (369) Sandstones at the base of the Elvins formation, in the eastern 

 limits of the town of Flat River, St. Francois County, Missouri. 



(69) Limestones near Honey Creek; and (70) limestone near Morgans Creek; both in Burnet County, Texas. 



Lingulella toerentis (Matthew). 

 Plate XXXV, figure 7. 



Leptobolus torrentis Matthew, 1903, Geol. Survey Canada, Rept. Cambrian Rocks Cape Breton, pp. 74-75, PL VI, 

 fig. 1. (Described as below as a new species. The specimen represented by fig. 1 is redrawn in this monograph, 

 PL XXXV, fig. 7.) 



Leptobolus atavus tritavus Matthew, 1903, idem, p. 109, PL VI, figs. 5a-c. (Characterized as a new variety.) 



The original description by Matthew follows: 



Shell thin, surface shining. Form elongate oval. 



Ventral valve obtusely pointed at the back, somewhat acutely rounded in front; elevated along the middle; some- 

 what flattened along the sides within the margin. 



By decortication a low boss and a transverse furrow behind it are exposed on the mold, about three-quarters of the 

 length of the valve from the hinge; if this boss marks the front of the callus, the central muscles are unusually far 

 forward, more advanced even than in L. atavus of the Etcheminian terrane. There are faint impressions of the lateral 

 septa on the sides of the valve. 



The surface of the shell is covered with minute, low tubercles, cancellate in arrangement; through these can be 

 traced faint parallel lines, concentric to the umbo. Along the median third in places can be seen about six broad, 

 flat ridges, radiating from the direction of the umbo; these break the continuity of the concentric ridges. 



Length, 3 mm.; width, 2 mm.; depth, 0.5 mm. 



The ventral valve of "Leptoholus atavus tritavus" Matthew is similar in form to the type of 

 Lingulella torrentis. Compare figures 1 and 5a of Matthew [1 903, PI. VI]. The surface characters 



