526 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



and Whitfield. In general, G loganensis is almost exactly similar to C.illinois- 

 ensis Worthen. Hall and Whitfield say (loc. cit, p. 254): "The species 

 resembles somewhat C. illinoisensis Worthen, from the Burlington limestone, 

 in the size and convexity of the valve, and also in the striae, but differs in 

 the greater proportional length of the hinge line and in the flattening of the 

 mesial portion." 



Chonetes loganensis is described and figured with submucronate alar 

 extensions, but this character is a restoration, for the type specimen is imper- 

 fect; and an incorrect one, for a study of considerable material, including 

 Hall and Whitfield's, as well as my own, shows that the prevailing form is 

 subcircular with parallel rectilinear sides which meet the hinge line at nearly 

 a right angle, the corners being usually either rectangular or rounded. 



The mesial flattening referred to is well shown in the type specimen, 

 but it is not a constant character, and, in fact, the curvature seems to be 

 most often regular and even. On the other hand, specimens of C. illinois- 

 ensis Worthen, from one of the type localities, shows exactly the same range 

 of variation, some having a distinct sinus or flattening, just as has been 

 observed in C. loganensis. In general I have been unable to discover any 

 constant differences which might distinguish the two species. 



The surface ornamentation of C. loganensis, wherever shown on areas 

 which are not, as usual, badly exfoliated, consists of fine, flexuous, often 

 bifurcate, radiating striae, which are rather angular and are separated by 

 interspaces about equal to their own diameter. These are crossed by very 

 fine, threadlike, concentric striae. 



The average size of mature shells is: breadth, 14 mm.; length, 10 mm.; 

 but the dimensions run up as high as 16 mm. in breadth by 11.5 mm. 

 in length. The radiating striae number about one hundred and fifty. 



It is impossible to confuse this with the only other species of Chonetes 

 known from the Yellowstone National Park. The radiating striae are finer 

 and much more numerous, the shell itself is less tumid, and the outline 

 is more quadrate. 



Formation and locality: Madison limestone, Hunter Peak, Absaroka 

 Range; White Mountains, Absaroka Range, just below Quadrant quartzite; 

 T. A. Jaggar. East side of Gallatin River, west of Electric Peak ; east 

 of Antler Peak, Grallatin Range; saddle west of Antler Peak, Grallatin 

 Range; amphitheater east of Bannock Peak, Grallatin Rang-e, bed 27; 



