552 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



near the anterior margin. In the variety madisonensis the striae bifurcate 

 near the anterior margin, but can be traced backward to their origin at the 

 beak without bifurcation. In neither form are the striae interrupted by 

 imbricating lamellae. 



Hall and Whitfield distinguish two types among the forms from Utah, 

 which they refer to S. striata s. The one which is lower in the stratigraphic 

 series is said to be transversely elongated, while the higher one is about as 

 long as wide, and has finer, more angular, and more fasciculate striae. 

 Their material is so crushed that it is impossible to institute a satisfactory 

 comparison, but if S. striatus var. madisonensis is identical with either of 

 these, it must be with the former (lower) type. It differs distinctly from 

 the form figured on PI. V, figs. 13, 14 (loc. cit), which is somewhat larger; 

 the area, instead of being nearly straight and vertical, is deepl} T curved, at 

 first nearly horizontal, then resurgent; the fold is sudden, high, and thin. 



Spirifer striatus var. madisonensis, when mature, can be readily distin- 

 guished from S. centronatus, with which it is associated. It is much larger, 

 the fold and sinus less defined, plications more numerous, but less sharp, 

 and not covered with the imbricating concentric lamellae which well- 

 preserved surfaces of S. centronatus show. 



The largest specimen, an incomplete ventral valve, measures 55 mm. 

 in breadth. Another shell measures 55 mm. in breadth by 35 mm. in 

 length, which is perhaps near the average. Young specimens, however, 

 are broader in proportion to their length, and can scarcely be distinguished 

 in shape from S. centronatus of the same size. 



Formation and locality: Madison limestone, Stinkingwater Valley, 

 below mouth of the canyon, Absaroka Range; Arnold Hague. Crowfoot 

 Ridge, Gallatin Range, cherty limestone, top of bed 24; A. 0. Gill. 



Spirifer sp. 

 PI. LXX, fig. la. 



This form is known only from an incomplete dorsal valve, but it is so 

 striking that it seems worthy of some notice. It is very transverse, the 

 width being 51 mm. and the length 16.5 mm. Shape triangular; trun- 

 cate in front; wings acutely angular, mucronate (?). The fold is unde- 

 fined and scarcely at all raised above the general curvature of the shell, 

 surmounted by four or five bifurcated plications. On either side are found 



