STREPTORHYNCHUS 73 



specimen is not similar to the strongly ferruginous casts of shells which 

 commonly come from that locality, and the specimen has rather the lith- 

 ologic character of some of the St. Louis limestone cherts. Furthermore, 

 in the collections examined, the species has been detected most commonly 

 in the typical St. Louis limestone. These St. Louis limestone specimens 

 agree well with the holotype of the species in the general form of the 

 shell, in the size of the costae, and in the extraordinary irregularity or dis- 

 tortion of the shell, but in no two examples are the irregularities of the 

 shell form alike. In some specimens the umbonal region is much flat- 

 tened, as in the type, while in others it is elevated and terminated by a 

 pointed and more or less distorted beak. The convexity of the brachial 

 valve is also a variable feature. An occasional example is found hav- 

 ing a minimum amount of irregularity of shell growth. On grinding 

 down the beak of several limestone examples, no median septum has been 

 detected, which confirms the observation made upon the type specimen. 

 The most important specific characters of the shell seems to be its irregu- 

 larity of growth, although the proportional length of the hinge-line is 

 rather constant, as well as the general outline of the shell. 

 Horizon. Salem limestone, St. Louis limestone and Batesville sandstone. 



STREPTORHYNCHUS ULRICHI Hall and Clarke 

 Plate II, Figs. 17-18 



1892. Streptorhynchus Ulrichi Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, 



p. 351, pi. 11B, fig. 15. 

 1897. Streptorhynchus Ulrichi Hall, 14th Ann. Rep. N. Y. State Geol., 



p. 350, pi. 6, fig. 3. 



Description. Shell below medium size, longer than wide, growing un- 

 symmetrically and distorted, marked by strong, irregular, concentric 

 lines of growth; the hinge-line shorter than the greatest width of the 

 shell, the cardinal extremities angular, the greatest width anterior to the 

 middle of the shell. The dimensions of the holotype, a pedicle valve, 

 are : length from front margin to beak 22 mm., length from front margin 

 to hinge-line 15 mm., greatest width 20 mm., length of hinge-line 16 mm., 

 depth 6 mm. 



Pedicle valve subtrihedral in general form, the surface somewhat flat- 

 tened medially from the beak towards the front, but without a distinct 

 sinus, sloping rather abruptly towards the postero-lateral margins and 

 sometimes a little compressed near the cardinal extremities, becoming de- 

 pressed towards the anterior margin in the holotype; beak prominent, 

 distorted and produced posteriorly far beyond the hinge-line; cardinal 

 area high, concave and somewhat distorted, the lateral margins sharply 

 defined ; delthyrium higher than wide, closed by a convex deltidium. In- 

 ternally the hinge-teeth are supported by dental lamellae which are pro- 



