348 MISSISSIPPIAN BRACHIOPODA 



SPIRIFER LITTONI Swallow 



Plate L, Figs. 1-5 

 1860. Spirifer Littoni Swallow, Trans. St. Louis Acad. Sci., vol. 1, p. 646. 



Description. Shell below medium size, longer than wide, the hinge-line 

 shorter than the greatest width, the cardinal extremities rounded. The 

 dimensions of a nearly perfect specimen are : length of pedicle valve 20.4 

 mm., length of brachial valve 17 mm., greatest width 23 mm., length of 

 hinge-line 16 mm., thickness 15.3 mm., height of cardinal area 3.5 mm. 



Pedicle valve rather strongly convex, the greatest depth towards the 

 umbo, the surface curving abruptly to the cardinal margin, more gently 

 to the lateral and anterior margins, not compressed towards the cardinal 

 extremities ; beak small, pointed and rather strongly incurved ; the car- 

 dinal area of moderate height, rather strongly arched, the curvature be- 

 coming greater towards the beak, the lateral margins not very sharply 

 defined, the delthyrium large, about as wide as high ; mesial sinus origin- 

 ating at the beak as a narrow, but well-defined depression, becoming 

 rather broad and moderately deep to the front where it is somewhat pro- 

 duced in an anterior extension of the valve, it is marked by a mesial pli- 

 cation which originates near the beak as a very fine, elevated line, becom- 

 ing rather broadly rounded, but not very high towards the front, and by a 

 single, fainter, sometimes nearly obsolete plication, upon each side, which 

 originates from the inner margin of the lateral bounding plications; the 

 lateral slopes regularly convex, each marked by from eight to ten simple, 

 rounded plications which grow successively smaller towards the cardinal 

 extremities. 



Brachial valve subelliptical in outline, its convexity about equal to or 

 a little less than that of the pedicle valve, its greatest depth near or pos- 

 terior to the middle, the umbonal region rather prominent and projecting 

 beyond the cardinal margin, the surface curving abruptly to the cardinal 

 margin and more gently to the lateral and anterior margins, slightly com- 

 pressed towards the cardinal extremities ; cardinal area narrow ; the beak 

 rather strongly incurved ; mesial fold originating at the beak as a flat- 

 tened, scarcely elevated plication, it is well defined and becomes mod- 

 erately elevated in front, with a rather broadly rounded mesial depres- 

 sion and usually a single fainter depression upon each side ; the lateral 

 slopes marked by simple, rounded plications entirely similar to those of 

 the opposite valve. 



The finer surface markings consist of rather inconspicuous concentric 

 lines of growth, an occasional one of which is somewhat stronger than the 

 others, and by an exceedingly fine reticulate marking which is only vis- 

 ible upon perfectly preserved surfaces. 



