444 Trans. Acad. Sci. of St. Louis. 



of the ribs. This specimen agrees closely with typical Ham- 

 ilton individuals of the species of the same size, or with the 

 corresponding portion of the shell in larger individuals. 



Spirifer jeffersonensis n. sp. 



Plate 1, Jigs. 18-22. 



Description. Shell of medium size, wider than long, the 

 cardinal extremities rounded, the hinge-line a little shorter 

 than the greatest width. Pedicle valve strongly convex, the 

 beak incurved : cardinal area high, concave, the cardinal mar- 

 gins rounding into the lateral slopes of the shell, the delthy- 

 rium broadly triangular; sinus moderately shallow, rounded 

 in the bottom, well defined at its sides, usually marked by 

 two more or less indistinct, sometimes nearly obsolete plica- 

 tions; lateral slopes slightly concave or convex from the 

 umbo to the cardinal extremities, convex antero-laterally. 

 Brachial valve moderately convex: the fold sharply defined, 

 but little elevated above the general surface of the valve in 

 the posterior half, sometimes becoming more strongly ele- 

 vated towards the front, indistinctly marked by three or four 

 low, rounded plications which are often apparently obsolete; 

 lateral slopes convex from the umbo to the antero-lateral 

 margins, curving more abruptly to the cardinal margin. Each 

 valve marked by from eight to twelve, usually nine or ten, 

 simple rounded plications upon each of its lateral slopes ; the 

 entire surface also marked by fine concentric lines of growth. 



The dimensions of a nearly perfect pedicle valve are: width 

 22.5 mm., length from beak to anterior margin 18 mm., 

 height of area 7 mm., width of delthyrium at hinge-line 

 7 mm. The dimensions of a brachial valve are : width 22 mm., 

 length 15 mm., convexity 6 mm. 



Remarks. At first sight this Spirifer, with its elevated 

 cardinal area and the inconspicious plications of the fold and 

 sinus, suggests the genus Syringothyris, but it lacks the 

 punctate shell structure of that genus and also the character- 

 istic syrinx. The condition of the plications of the fold and 

 sinus is a variable character, in a few of the specimens ex- 

 amined they are perfectly evident, in others they are nearly 

 obsolete and in a few brachial valves they seem to be entirely 



