122 MISSISSIPPIAN BRACHIOPODA 



tions at the cardinal extremities rather small and somewhat inflated ; the 

 beak strongly incurved beyond the hinge-line ; the mesial sinus absent 

 or very slightly developed. Surface of the valve marked by fine, rounded, 

 radiating costae, about four of which occupy the space of two millimeters 

 towards the front of the shell, posteriorly the costae increase by bifur- 

 cation, but beyond the middle of the valve they continue to the front 

 margin either with none or with infrequent divisions; crossing the 

 radiating costae on the posterior portion of the valve there are rather 

 inconspicuous, wrinkle-like, concentric markings which are strongest 

 near the auriculations ; the entire surface also marked by extremely fine 

 concentric lines of growth which are often obliterated by exfoliation 

 of the surface. Spine bases scattered more or less sparsely over the 

 surface of the valve with a regularly arranged row usually or always 

 present upon each side of the beak, starting close to the hinge-line and 

 describing a curve around the base of the auriculation to the lateral 

 margin a short distance in front of the cardinal extremity, the spines 

 continuously becoming stronger distally. Internal characters not ob- 

 served. 



Brachial valve deeply and rather regularly concave, produced ante- 

 riorly and laterally nearly in contact with the opposite valve, the auricu- 

 lar portions at the cardinal extremities somewhat differentiated from the 

 general surface. Surface marked by radiating costae similar to those of 

 the opposite valve and also .by concentric markings ; spine bases appar- 

 ently infrequent or absent. Internal characters not observed. 



Remarks. This species is characterized by its small size, its broad um- 

 bonal region, and by the arrangement of the spine bases in a row around 

 the bases of the auricular extension of the pedicle valve. The typical 

 form of the species occurs in the Chester, but a form from the Pella beds 

 of Iowa has also been referred here, which shows some variation from 

 the typical form. In size, general form and proportions the two shells 

 are essentially alike except that some of the Pella specimens are a little 

 larger. The chief difference lies in the greater number of spine bases 

 upon the Pella examples over the entire surface of the pedicle valve, and 

 in the crowding of a group of them upon the auriculations outside of the 

 row which encircles the auriculation. 



Horizon. Ste. Genevieve limestone and Chester group. 



PRODUCTUS SETIGERUS Hall 

 Plate XII, Figs. 1-3 



1858. Productus setigerus Hall, Geol. Iowa, vol. 1, pt. 2, p. 638, pi. 19, 



fig. 3. 



1883. Productus semireticulatus Hall, Kep. N. Y. State Geol. for 1882, 

 pi. (18) 49, fig. 13 (not figs. 11 and 12). 



