112 MISSISSIPPIAN BRACHIOPODA 



margin 28 mm., length of hinge-line 36 mm., width of body of shell at its 

 mid-length 29.3 mm., convexity of pedicle valve 17 mm. 



Pedicle valve gibbous, the umbonal region conspicuously protuberant 

 beyond the hinge-line, the beak strongly incurved, the surface curving 

 abruptly to the cardinal margin and a little more gently to the lateral 

 and anterior margins, auriculate at the cardinal extremities, the auricula- 

 tions not sharply differentiated from the lateral slopes, rather high and 

 arched antero-posteriorly with the curvature a little more abrupt to the 

 cardinal margin; mesial sinus originating in the umbonal region, rather 

 narrow and of moderate depth. Surface of valve marked by rounded, 

 radiating costog which increase by bifurcation on the posterior slope of the 

 valve, usually continuing across the anterior slope without division, 

 tending to become fainter or nearly obsolete towards the margin, from 

 .5 mm. to 1 mm. apart in front, usually a little less than 1 mm., becoming 

 smaller and more crowded posteriorly and laterally ; crossing the radiat- 

 ing costaa upon the posterior slope of the valve are concentric, wrinkle-like 

 markings which give to that surface a semicirculate ornamentation ; spine 

 bases apparently absent or very sparsely and irregularly scattered upon 

 the body of the shell, but when the shell is w r ell preserved a row of three 

 or four rather strong spine bases is present near and parallel with the car- 

 dinal margin on each side of the beak. Internal characters of the valve 

 not observed. 



Brachial valve only imperfectly known, it is moderately concave in 

 its posterior portion, leaving a rather deep visceral cavity between the 

 two valves, becoming more strongly curved anteriorly and laterally and 

 approaching closely the opposite valve. The surface markings are similar 

 to those of the opposite valve. 



Remarks. In its general size and form this species approaches some- 

 what closely to P. burlingtonensis. It may be easily distinguished from 

 that species by its less depressed or more inflated auriculations of the 

 pedicle valve, with the row of spine bases along the cardinal margin, by 

 the narrower and somewhat more sharply defined mesial sinus and by the 

 somewhat finer radiating costas. The row of spine bases along the cardinal 

 margin is commonly difficult to recognize because of the condition of 

 preservation of the shells, but the inflated auriculations are easily dis- 

 tinguished in almost every example observed. 



Horizon. Chester group, Okaw formation. 



PRODUCTUS MESIALIS Hall 

 Plate X, Figs. 7-13; Plate LXXXIII, Figs. 14-17 



3858. Productus mesialis Hall, Geol. Iowa, vol. 1, pt. 2, p. 636, pi. 19, 



figs. 2a-c. 

 1883. Productus mesialis Hall, Rep. N. Y. State Geol. for 1882, pi. (18) 



49, figs. 9-10. 



