116 FLOEA OF LOWER COAL MEASURES OF MISSOURI. 



as the mature pinua?. The nerves are mostly simple, never forking- more 

 than once, nearly straight, and very open and distant, the midrib thick, and 

 the pinnules very irregular, all trie diagnostic features being those charac- 

 teristic of the ordinary jnnnse. On the back of the rock containing the 

 original of the figure of AJathopteris Serlii Brongn., fig. 2, pi. xxix, of the 

 Coal Flora, there is a fine specimen of Alethopteris ambigua, so labeled by 

 Professor Lesquereux, containing the upper segment, about 18 cm. in 

 length, of a primary pinna of the true form. Here we have the long 

 pinujB, like those on the left of the type of fig. 1, pi. xxxi, of the Coal 

 Flora, succeeded in passing upward by pinnae of the character of our Fig. 

 3, PI. XXXVII, while at the top we have the long, slightly pinnatifid and 

 crenulate pinnules, like those in the lower right-hand pinnse of the figure 

 in the Coal Flora. 



x4.nother specimen, apparently belonging to this species, loaned by 

 Dr. Britts, covers the surface of a slab 26 cm. long and 25 cm. wide. In 

 this we have a rachis 7 mm. wide, slightly flexuous, and giving off alternating 

 compound pinnae at intervals of 3-8 cm. on each side, some of these second- 

 ary pinnse beii:ig shown in their entire length. The whole segment comes 

 apparently from some distance below the apex of a primary pinna. Unfor- 

 tunately the specimen is somewhat shriveled throughout most of its extent. 



That the ultimate pinnae of this species were often long and slender 

 is shown by the fact that it is very difficult to obtain entire specimens, 

 although some incomplete fragments were found measuring 18 cm. or more 

 in length and less than 25 mm. in width. 



One fragment which seems inseparable from this species presents in 

 the appearance of its flattened border, as well as by its ensemble of char- 

 acters, a striking resemblance to the figure of Alethopteris Gibsoni Lx. given 

 in fig-. 6, pi. xxviii, of the Coal Flora, and I am not at present sure that the 

 latter does not represent a specimen belonging more properly to our species. 



Alethopteris ambigua is easily distinguishable, by the characters recited 

 above, ivom. Alethopteris aquilina, which is, with the exception oi Alethopteris 

 Gibsoni, probably the nearest related species of Alethopteris, and the only 

 one with which it is likely to be confused. 



Localities. — Common at Owen's coal bank, U. S. Nat. Mus., 3590, 

 5490; rare at Gilkerson's Ford, U. S. Nat. Mus., 3592, 5488; Deepwater, 

 U. S. Nat. Mus., 3591, 3593; Pitcher's coal bank, U. S. Nat. Mus., 3634. 



