ECHINOCONCHUS 141 



Remarks. This species is much like E. altematus of the Keokuk lime- 

 stone, but mature examples are always much smaller than in that 

 species, the hinge-line is proportionally shorter, the umbonal region is 

 much narrower and more sharply pointed towards the beak. These 

 characters are apparently constant and the shell is so commonly found 

 in the Ste. Genevieve limestone that it has seemed best to recognize 

 it as a good species, although a broad interpretation of E. punctatus might 

 include not only this form but E. alternatus as well. 



Horizon. Ste. Genevieve limestone. 



ECHINOCONCHUS BISERIATUS (Hall) 

 Plate XVII, Figs. 10-15 



1855. Productus fimbriatm Norwood and Pratten, Jour. Acad. Nat. Sci. 



Phil. (2), vol. 3, p. 19 (not P. fimbriatus Sowerby). 



1856. Productus biseriatus Hall, Trans. Albany Inst., vol. 4, p. 12. 



1882. Productus biseriatus Whitfield, Bull. Am. Mus. Nat. Hist., vol. 1, 



pi. 6, figs. 8-12. 



1883. Productus biseriatus Hall, 12th Rep. Geol. Surv. Ind., p. 325, pi. 



29, figs. 8-12. 



1894. Productus biseriatus Keyes, Mo. Geol. Surv., vol. 5, p. 43. 

 1906. Productus biseriatus Beede, 30th Ann. Rep. Geol. Surv. Ind., p. 



1299, pi. 19, fig. 6 ; pi. 22, figs. 8-12. 

 1911. Productus biseriatus Girty, Bull. U. S. Geol. Survey No. 439, p. 46, 



pi. 3, figs. 10-11. 



Description. Shell small, length and width usually about equal, but 

 varying from longer than wide to wider than long, the hinge-line 

 shorter than the greatest width, the greatest width near or anterior to 

 the mid-length, the cardinal extremities obtusely angular or a little 

 rounded. The dimensions of a nearly perfect specimen are : length 

 from hinge-line to front margin 11.4 mm., length from umbonal region 

 of pedicle valve to front margin 13.7 mm., greatest width 13.5 mm., 

 length of hinge-line 10 mm., convexity of pedicle valve 7 mm., depth 

 of visceral cavity between the two valves 4.4 mm. 



Pedicle valve strongly convex, the umbonal region pointed and pro- 

 tuberant beyond the hinge-line, the surface curving abruptly to the 

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

 pressed at the cardinal extremities to form small auriculations which 

 are not sharply differentiated from the lateral slopes; mesial sinus obso- 

 lete ; beak pointed and strongly incurved beyond the hinge-line. Surface 

 marked by regular concentric bands which are differentiated by a slight 

 elevation of the surface along their anterior margin, upon each concentric 

 band is a row of elongate nodes, highest in front, the bases of appressed 

 spines, which become lower posteriorly and disappear in front of the 

 posterior margin of the band; closely crowded between and in front of 

 these larger nodes are numerous finer but entirely similar nodes which 



