DELTHYRIS 301 



early Mississippian faunas which are apparently wholly devoid of punctate 

 shell structure and must consequently be excluded from Spiriferina, and 

 the genus Delthyris is apparently the only place where they can be prop- 

 erly located. The only essential difference between Delthyris and Spiri- 

 ferina, as interpreted by Schuchert, 1 and followed by the author, 

 is in the absence of punctation in Delthyris. Both genera are characterized 

 by the well developed median septum in the pedicle valve, by the simple 

 plications of the shell, by the strong, concentric, lamellose lines of growth, 

 and by the essentially spiriferoid external form. It is believed that the 

 shells here placed in the genus are actually devoid of punctations and 

 that their absence is not due to the condition of preservation since the 

 structure can always be detected to some extent, at least, in all the un- 

 doubted Spiriferiiias, whatever their condition of preservation, but in 

 these shells it has never been detected. 



DELTHYRIS CLARKSVILLENSIS (Winchell) 

 Plate XXXVI, Figs. 6-7 



1865. Spiriferina, Clarksvillensis Winchell, Proc. Acad. Nat. Sci. Phil., 



p. 119. 



1894. Spiriferina clarkesvillensis Keyes, Mo. Geol. Surv., vol. 5, p. 85. 

 1908. Spiriferina clarksvillensis Rowley, Mo. Bureau Geol. and Mines, vol. 



8, 2nd ser., p. 83, pi. 18, figs. 10-12. 



Description. Shell below medium size, wider than long, hinge-line as 

 long as, or a little shorter than, the greatest width of the shell, the cardinal 

 extremities angular or a little rounded. The dimensions of a nearly com- 

 plete but somewhat distorted specimen are: length of pedicle valve 11 

 mm., length of brachial valve 9.1 mm., width 16.8 mm., thickness 8 mm., 

 width of sinus in front 5.4 mm., height of cardinal area 3.9 mm. 



Pedicle valve moderately convex, most prominent on the umbo, but 

 little or not at all compressed towards the cardinal extremities, the sur- 

 face curving rather abruptly to the cardinal margin on each side of the 

 beak, more gently to the anterior margin and most gently to the lateral 

 margins ; mesial sinus rather wide, of moderate depth, non-plicate, and 

 rounded in the bottom ; beak only moderately incurved ; cardinal area of 

 moderate height, nearly flat below, becoming concave towards the beak, 

 the lateral margins sharply defined or rounding slightly into the surface 

 of the lateral slopes ; the delthyrium of moderate size, higher than wide ; 

 each lateral slope marked by from four to seven, rather broad, rounded, 

 plications which originate along the cardinal margin. Internally a me- 

 dian septum is present which reaches between one-third and one-half the 

 length of the shell from the beak, the remaining internal characters not 

 observed. 



1 Synop. Am. Foss. Brach., Bull. 87, U. S. Geol. Survey, p. 206. (1897.) 



