SYRINGOTHYRIS 401 



margin of the valve, not deeply impressed, extending laterally beyond 

 the limits of the mesial fold posteriorly, but confined within the limits of 

 the fold anteriorly, marked by a slight median depression which is some- 

 times scarcely noticeable. 



Remarks. This is perhaps the best known species of Syringothyris, being 

 especially abundant in some of the Knobstone beds of southern Indiana. 

 It is characterized by the great width and the comparatively low and 

 nearly flat cardinal area, Schuchert 1 has considered both 8. subcuspidatus 

 and 8. propinquus as synonyms of 8. texta, but 8. subcuspidatus is certainly 

 distinct from it and perhaps also 8. propinquus. The distinguishing char- 

 acteristics of these species will be discussed under the descriptions of 

 these forms. 



Horizon. Keokuk limestone and Knobstone formation. 



SYRINGOTHYRIS SUBCUSPIDATUS (Hall) 

 Plate LXXI, Figs. 3-7 



1858. Spirifer subcuspidatus Hall, Geol. Iowa, vol. 1, pt. 2, p. 646, pi. 20. 

 figs. 6a-b. 



1894. Syringothyris subcuspidata Hall and Clarke, Int. to Study of Brach., 



pt. 2, pi. 30, fig. 3. 



1895. Syringothyris subcuspidatus Hall and Clarke, Pal. N. Y., vol. 8, 



pt. 2, pi. 26, figs. 8, 11 ; pi. 27, fig. 18. 



Description. Shell large, wider than long, the greatest width at the 

 hinge-line, the cardinal extremities usually sub-rectangular. The dimen- 

 sions of a nearly complete specimen are : length of pedicle valve from 

 beak to front 53.5 mm., length of brachial valve 40.3 mm., width 66 mm., 

 height of cardinal area 18.6 mm., thickness 42 mm., width of sinus in 

 front 26.2 mm., width of delthyrium at base 12 mm. 



Pedicle valve strongly convex, most prominent on the umbo, the surface 

 sloping to the cardinal extremities with a gently convex curvature, and 

 with a greater curvature to the antero-lateral and anterior margins ; 

 mesial sinus of moderate depth, rounded in the bottom, originating at 

 the beak where it is narrow and sharply defined, becoming broader and 

 less sharply defined anteriorly, produced in front in a rounded lingual 

 extension; the beak nearly erect or a little incurved, sometimes slightly 

 unsymmetrical or twisted ; cardinal area rather high, broadly triangular, 

 nearly flat or gently concave, sloping posteriorly from the hinge-line, the 

 inferior portion lying at an angle of about 95 degrees to the plane of the 

 valve^ the differentiated, vertically marked region lying on either side 

 of the delthyrium is very broad, the base of the triangle being more than 

 one-half of the total length of the hinge-line ; delthyrium broad, its width 



19th Ann. Rep. N. Y. State Geol., p. 34. (1890.) 



