SYRINGOTHYRIS 393 



SYRINGOTHYRIS MISSOURI Hall 

 Plate LXX, Figs. 6-15 



1894. Syringothyris Missouri Hall, 13th Ann. Rep. N. Y. State Geol.. 



p. 650. 



1895. Syringothyris Missouri Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, 



p. 363, pi, 39, figs. 29-31. 



1897. Syringothyris Missouri Hall, 14th Ann. Rep. N. Y. State Geol., p. 

 358, pi. 8, figs. 14-16. 



Description. Shell small, with very high cardinal area, wider than long, 

 the greatest width a little in front of the hinge-line, the cardinal extremi- 

 ties rounded. The dimensions of two specimens, the larger one the holo- 

 type of the species, are : length of pedicle valve from beak to front margin 

 14.8 mm. and 13.5 mm., length of brachial valve 11.5 mm. and 10.9 mm., 

 height of cardinal area 10 mm. and 9 mm., maximum width of shell 20 

 mm. and 18.5 mm., length of hinge-line 17 mm. and 14 mm., thickness 15 

 mm. and 13.5 mm., width of sinus in front 6 mm. and 5.2 mm. 



Pedicle valve subpyramidal in form, the surface sloping steeply from 

 the umbo to the lateral and anterior margins with a slightly convex curve 

 or with almost no curvature ; the mesial sinus originating at the beak, 

 shallow, non-plicate, rounded in the bottom, and rather sharply defined, 

 produced in front in a short, rounded extension; beak small, a little 

 curved; cardinal area very high, nearly flat, except close to the apex 

 where it is a little arched, the lateral margins ill-defined, the surface 

 rounding into the lateral slopes; delthyrium narrowly triangular, much 

 higher than wide, the point of the syrinx extending two-thirds the dis- 

 tance from the apex to the hinge-line ; each lateral slope marked by from 

 eight to ten simple, depressed, radiating plications which grow succes- 

 sively smaller towards the cardinal extremities. Internally the dental 

 plates are thin and well-developed, sometimes extending nearly half-way 

 from the beak to the front margin along the floor of the valve ; the char- 

 acter of the muscular scars is not clear in any of the specimens studied; 

 the outer surface of the transverse, syrinx-bearing plate, between the 

 dental lamellae is distinctly marked by a longitudinal median rib which 

 is extended in the free, spine-like portion of the syrinx towards the hinge- 

 line. 



Brachial valve moderately convex, a little compressed towards the car- 

 dinal extremities, the surface curving more abruptly to the cardinal mar- 

 gin; mesial fold originating at the beak, low, rounded on top, sharply 

 defined; beak short, extending a little beyond the hinge-line, incurved; 

 the cardinal area very narrow. 



External surface of the shell not preserved on any of the specimens 

 examined, but the surface of the exfoliated shells shows that the shell 

 substance was punctate. 



