452 GEOLOGY OP THE YELLOWSTONE NATIONAL PARK. 



rather small muscular area, and the interior of the dorsal (brachial) valve 

 a slightly elevated area upon which occurs a narrow, short median septum. 

 The crural plates are also well shown. In casts of the interior from the 

 St. Croix sandstone of Wisconsin the dental lamellse of the ventral valve 

 are finely shown, and in the dorsal valve the median septum and crural 

 plates. 



This is one of the most variable shells that occur in the Cambrian 

 fauna. Its range of variation is such in all of the widely separated locali- 

 ties in which it occurs that one would scarcely hesitate, if in possession 

 only of the extremes, to identify two well-marked species. The variation 

 is not only in the radiating costee, but also in the general form of the shell. 

 It is proposed to illustrate this variation somewhat fully in a memoir on 

 the Brachiopoda of the Cambrian fauna. 



Formation and locality : Near base of Upper Cambrian, Gallatin ter- 

 rane, Crowfoot section, Gallatin Range, Yellowstone National Park. It 

 also occurs at a slightly lower horizon on the south side of the Gallatin 

 Valley, and specimens were collected farther to the north by Dr. A. C. 

 Peale, opposite the mouth of Pass Creek, in the Gallatin Valley, Montana. 



Orthis (?) sandbergi Winchell. 



PI. LXI, figs. 2, 2a-d. 



Orthis sandbergi Wincliell, 1S86 : Fourteenth Ann. Kept. Geol. and Nat. Hist. Surv. 

 Minnesota, p. 318, PI. II, figs. S, 9. 



Shell small, transverse, subquadrate in outline, exclusive of the acute 

 angular ears. Valves slightly convex, with a straight hinge line longer 

 than the greatest width of the shell ; cardinal area narrow but well devel- 

 oped on each valve and divided by a rather large open clelthyrium. 



The ventral (pedicle) valve slightly flattened at the ears, rising toward 

 the center with a convex triangular swelling, broadening from the narrow 

 beak to the front ; beak small, rounded, and extending slightly beyond the 

 hinge line. Dorsal valve flattened at the ears, with well-marked rounded 

 ridges rising between the ears, and a rather broad, well-defined median 

 sinus ; beak very small, slightly encroaching upon the hinge line. 



Surface marked by fine, regular, radiating' striae, between which one or 

 more faint intermediate stria? are sometimes visible; under favorable con- 



