PEIMOBDIAL FOSSILS. 335 



BRACHIOPODA. 



LINGULID^L 

 Genus LINGULEPIS Hall. 



LINGULEPIS PINNAFOBMIS. 



Plate 2, figs. 1-4. 



Lingula pinnaformis Owen, Geol. Eept. Wis., Iowa, and Minnesota, p. 583, PI. 1 B, figs. 



4, 6, and 8. 

 Lingula pinnaformis Owen, Bepts. ofWisconsin, vol. 1, p. 21, fig. 3, and p. 435. 

 Lingula antiqua Hall, Foster & Whitney's Lake Sup., part 2, p. 204, PI. 23, fig. 2. 

 IAngulepis pinnaformis (Owen) Hall, 16th Bept. State Cab., p. 129, PL vi, figs. 14, 16. 



Shell elongate-spatulate, rounded in front and on the sides, becoming 

 narrowed and attenuate toward the beak, especially on the ventral valve, 

 the cardinal slopes of which are long and slightly concave; apex of the 

 valve very slightly truncate; width about three-fifths as great as the length 

 in the most perfect individuals; surface of the ventral valve beautifully 

 rounded over the body of the shell, becoming proportionally more sharply 

 elevated along the middle on the narrowed portion, and almost subangular 

 on the upper part of the beak. Dorsal valve shorter than the ventral, more 

 broadly truncate at the upper end, and less regularly rounded on the sur- 

 face, sometimes being slightly flattened in the lower part. 



Surface of the shell polished, but presenting decided lines of growth, 

 which, on weathered or partially exfoliated specimens, show a tendency to 

 a lamellose structure; they are also marked in the latter condition by fine 

 radiating lines, which are not apparent on the more perfect specimens. 



The examples of this species before us are so exactly similar in their 

 form and characters to those from the Falls of Saint Croix, that it is impos- 

 sible to point out any differences that can be described. 



Formation and locality. — In greenish-purple sandstone of the Potsdam 

 formation, at the headwaters of Red Canon Creek, Black Hills, Dakota. 



