SYRINGOTHYBIS 397 



upon the transverse delthyrial plate joining the dental lamellae. The 

 group of shells from which the syrinx is lacking includes several distinct 

 specific forms from various horizons, and is described as a new genus 

 (Pseudosyrinx] in the present report. Among the Waverly specimens 

 commonly referred to S. carteri, examples preserving the delthyrial char- 

 acters are rare, but among them both syrinx-bearing shells and others 

 without the syrinx have been observed, and as it seems to be impossible 

 to determine surely what S. carteri really is, the name should be dropped. 

 S. typa differs from S. hannibalensis in its larger size, its proportionally 

 higher cardinal area, in its more deeply placed and transversely concave 

 delthyrial plate, and much longer free extension of the syrinx. From the 

 European 8. euspidatus with which it has sometimes been compared, 8 

 typa differs in its lower cardinal area, which is concave instead of more 

 or less strongly convex, in the symmetrical or nearly symmetrical beak 

 of the pedicle valve instead of the more or less strongly twisted beak of 

 8. euspidatus, and in the very different condition of the syrinx, the free 

 portion of which is exceedingly elongate and slender in the European 

 species. 



Horizon. Upper Kinderhook. 



SYRINGOTHYRIS PLATYPLEURUS n. sp. 

 Plate LXXII, Figs. 1-4 



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

 along the hinge-line, the cardinal extremities acutely angular. The di- 

 mensions of the holotype are : length of pedicle valve from beak to front 

 34.5 mm., length of brachial valve 23 mm., width 60 mm., height of car- 

 dinal area 38.5 mm., thickness 41.8 mm., width of sinus in front 17 mm., 

 width of delthyrium at hinge-line 13 mm. 



Pedicle valve subpyramidal in form, the surface sloping in nearly 

 straight lines to the cardinal extremities, the lateral and anterior mar- 

 gins slightly compressed vertically towards the cardinal extremities ; 

 mesial sinus shallow, rounded in the bottom, not sharply defined ; the beak 

 erect, pointed; cardinal area very high, nearly flat, its lateral margins 

 sharply defined, sloping anteriorly from hinge-line at an angle of about 

 40 degrees to the plane of the valve, the triangular, vertically marked, 

 differentiated region lying on each side of the delthyrium with a width 

 at its base a little less than one-half the length of the hinge-line ; delthy- 

 rium narrowly triangular, nearly three times as high as wide ; each lateral 

 slope marked by about 20 rather faint, depressed plications which orig- 

 inate along the cardinal margin and grow successively smaller towards 

 the cardinal extremities. Internally the dental lamellae extend more 

 than half way from the beak to the anterior margin at the sides of the 

 sinus along the floor of the valve, their angle of divergence being essen- 



