OBOLIDiE. 401 



valve 11.5 mm. in length has a width of 11 mm. A dorsal valve 9 mm. in length has a width 

 of 9 mm.; another dorsal valve 11 mm. in length has a width of 12 mm. 



Casts of the interior of the ventral valve show a very clearly defined area of medium length 

 (PI. VIII, fig. Id) ; it is divided midway by the cast of a strong pedicle groove and about half- 

 way between the groove and the lateral margin by a distinct flexure line ; striae of growth cross 

 the area parallel with its front margin, being flexed on the flexure line and arching over the cast 

 of the pedicle groove. None of the casts show whether the area formed a shelf between the 

 pedicle groove and the lateral margins, as in most species of the genus Lingulella. The area of 

 the dorsal valve is short and relatively narrow, extending about halfway out on the cardinal slopes. 



The cast of the visceral cavity of the ventral valve is well shown by several specimens 

 (PI. VIII, figs. Id and le). It resembles that of Oiolus apoUinis quenstedti (Mickwitz) in having 

 the anterior margins extending almost directly outward from the center toward the impressions 

 of the main vascular sinuses. There are no traces of a median septum in the ventral valve; in 

 the dorsal valve it is shown in the cast as a very narrow depression and between and a little for- 

 ward of the central muscle scars (PL VIII, fig. 111). The thickening of the shell at the anterior 

 margin of the visceral cavity of the ventral valve is quite marked, as shown in Plate VIII, 

 figure If. 



The muscle scars are well shown in one specimen of the ventral valve (PI. VIII, fig. If). 

 The central (h), middle (k), and outside lateral (1) scars occur, but usually the scars can not be 

 differentiated. The anterior lateral (j) and transmedian (i) scars are also quite distinctly 

 ' shown. In the dorsal valve (PI. VIII, fig. Ih) the central (h) and anterior lateral (j) scars are 

 clearly defined, also the transmedian (i) scars. Of the markings left on the shell by the vascular 

 system the main vascular sinuses are, usually well shown in the casts, and in one specimen of the 

 ventral valve both the inner and lateral branches are finely outlined (PI. VIII, fig. Id). 



The parietal scar (ps) is nearly transverse in front of the visceral area of the ventral valve; 

 it arches forward at the center and curves a little backward toward the main vasciilar sinuses. 

 Its course outside of the latter is partly shown by Plate VlII, figure Id. In the dorsal valve its 

 course may be followed from the median line in front of the anterior lateral scars to the outward 

 cm-ve of the trunk sinuses, beyond which it can not be traced with any degree of certainty. 



Ohservations. — This species has not heretofore been well described or illustrated. Hall 

 [1863, p. 130] noticed that there were certain ovate shells associated with "Lingulepis pinna- 

 formis" which had a smooth, glossy surface, with fine concentric striae, and that when the outer 

 surface was removed it was very distinctly striated by fine, somewhat bifurcating striae. In 

 accordance with the system in vogue at that time he proposed, in event of the form proving 

 to be a distinct species, to have it called Lingulepis matinalis. A smaller shell occm's in asso- 

 ciation with Dicellomus politus (Hall) at Eau Claire, Wisconsin, that appears to be identical 

 with 0. matinalis. It differs in having smaller central muscle scars in the dorsal valve, but, as 

 the variation in form and size of the muscle scars is often considerable in shells of the same 

 species, this is not considered sufficient for specific differentiation. 



The Texas shells from Packsaddle Mountain (PI. VIII, figs. 1 l-o) are smaller than the 

 typical Wisconsin forms, and are identified with Obolus tetonensis ninus (PI. XI, figs. 1, la-g). 

 The specimens represented in Plate VIII, figures lb, le, and Ih, are larger and more readily 

 identified with the species. A single ventral valve from the Bighorn Mountains is similar in 

 form to ventral valves from Texas. At La Crosse, Wisconsin, the shells are smaller and occin' 

 about 200 feet below the base of the arenaceous limestone of the Ordovician. 



Tliis species appears to be distinct from any described form. It approaches most nearly 

 to 0. msera (Hall and Whitfield), differing from the latter in being slightly less elongate. 



Formation and locality. « — Lower Ordovician: (360a) Red siliceous limestone on west side of Trout Creek below 

 Bergen Park, 7 miles (11.2 km.) north-northwest of Manitou, El Paso County, Colorado. 



Upper Cambrian: (302d) Limestone 200 yards (183 m.) north of the southwest corner of sec. 18, T. 28 N., R. 113 W., 

 Uinta County, Wyoming. 



1 Owen [1852, p. 583] gives the type locality as "tails of the St. Croix, Minnesota;" this might be compared with our localities 328h, 82s,. 

 and 328e, all in Wisconsin. 



62667°— VOL 51, pt 1—12 26 



