. MESOZOIC FOSSILS. 615 



slightly flattened by pressure and somewhat exfoliated. The radiating 

 striae are barely visible under a lens, and they are considerably exaggerated 

 in the original enlarged figure. It differs from typical examples of C. 

 hellistriatus in its more slender form and smoother surface, the radiating stria?, 

 being almost obsolete, though it is sometimes difficult to determine whether 

 this is a natural feature or due to exfoliation. The doubt as to its identity 

 with C. extenuatus has already been mentioned. 



In the Park it is most common in the upper fossiliferous band of the 

 Jura, occurring on Gardiner River southeast of Electric Peak; south slope 

 of ridge south of Gray Mountain; saddle west of south head of Gardiner; 

 west of Snake River, 4 miles south of second crossino ; top of hill 3 miles 

 southeast of Gravel Peak, northwest of Flat Mountain; ridge south of 

 Mammoth Hot Springs, on main terrace; east slope of Mount Sheridan, and 

 ridge south of Mount Sheridan. 



Camptonectes platessiformis White. 



Camptonectes platessiformis White, 1876: Geol. Uinta Mountains, p. 93. 1880: Ann. 



Kept, U. S. Geol. Surv. Terr, for 187S, p. 143, PI. XXXVII, fig. 5a. 

 Camptonectes extenuatus (M. and H.) Whitfield, 1880: Geol. Black Hills Dakota, p. 353, 



PI. IV, fig. 4. 

 Xot Camptonectes f extenuatus Meek and Hayden, 1865: Palfeont. Upper Missouri, 



p. 78, PI. Ill, fig. 6. 



This species is more slender than C. hellistriatus, the height from beak 

 to base being considerably greater than the length, and it is apparently 

 somewhat more convex. The sculpture is coarser than in the typical form 

 of that species, but the radiating lines are somewhat more closely arranged 

 than in the variety distans. The most important difference, however, is in 

 the ears, which in the left valve are very large and have the form of right- 

 angled triangles, so that the hinge line is almost as long as the greatest 

 length of the shell. No good specimens of the right valve have been seen, 

 but an internal cast from the lower canyon of the Yellowstone shows that 

 it has a deep byssal sinus, and that the posterior ear is nearly rectangular, 

 instead of having the very oblique form seen in C. hellistriatus. From C. 

 pertenuistriatus this species may be easily distinguished by its much coarser 

 sculpture and by differences in outline. 



The specimen figured by Whitfield in the Geology of the Black Hills 

 as C. extenuatus is clearly identical with C. platessiformis, as may be seen 



