PARYPHORHYNCHUS 189 



there is no cardinal process, the hinge-plate is divided to the base and 

 its initial portion is supported by a divided median septum forming a 

 crural cavity at the base of the divided hinge-plate, the median septum 

 continues with gradually decreasing height for about one-fourth the length 

 of the valve from the beak. 



In addition to the plications the surface is marked by fine radiating 

 striae, about four of which occupy the space of one millimeter, and by 

 concentric lines of growth which are irregular in strength and distribu- 

 tion but usually become more conspicuous anteriorly. 



Remarks. This species, so far as known, is restricted in its distribution 

 to some of the fine yellow sandstone formations in the Kinderhook at 

 Burlington, Iowa; Washington County, Iowa; and Kinderhook, Illinois. 

 It is miost closely allied to P. striaticostatum, but it attains a larger size 

 than that species and is proportionally much wider with a more obtuse 

 beak. Some of the young individuals have more the outline of P. 

 striaticostatum, but as they increase in age they become more transverse. 



Horizon. Chonopectus sandstone of the Kinderhook. 



PARYPHORHYNCHUS STRIATICOSTATUM (Meek and Worthen) 



Plate XXVI, Figs. 15-22 

 1868. Rhynchonella Missouriensis Meek and Worthen, Geol. Surv. 111., vol. 



3, p. 450, pi. 14, figs. 7a-d. 

 1868. Rhynchonella striatocostata Meek and Worthen, Geol. Surv. 111., 



vol. 3, p. 452. 

 1895. Pugnax striato-costata Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, 



p. 204. 

 1895. Pugnax missouriensis Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, 



pi. 60, figs. 33-34. 

 1901. Pugnax striatocostata Weller, Trans. St. Louis Acad. Sci., vol. 11, 



p. 154, pi. 13, figs. 14-16. 

 1905. Paraphorhynchus striatocostatum Weller, Trans. St. Louis Acad. 



Sci., vol. 15, p. 262, pi. 1, figs. 6-11. 



Description. Shell of about medium size, broadly subovate or subtrian- 

 gular in outline, the width usually greater than the length, the greatest 

 width near or anterior to the mid-length, the postero-lateral margins 

 slightly convex or nearly straight, meeting at the beak in an angle of 

 from 80 to 100 degrees, the antero-lateral and anterior margins regularly 

 rounded or with the anterior margin sometimes straightened. The dimen- 

 sions of a nearly perfect specimen are: length 25 mm., width 26 mm., 

 thickness 17.8 mm., width of sinus in front 18 mm. 



Pedicle valve shallow, the surface flattened towards the umbo, curving 

 abruptly to the postero-lateral margins and sometimes becoming some- 

 what inflected, curving gently to the antero-lateral margins and strongly 

 arched from beak to front ; mesial sinus originating on or close to the um- 

 bonal region, becoming deep and broad anteriorly, rounded in the bottom 



