RHYNCHOPORA 237 



Brachial valve much more strongly convex than the pedicle, becoming 

 ventricose towards the front, the umbonal region mesially flattened 

 or with a shallow, longitudinal depression which sometimes continues to 

 the front of the valve, the surface sloping with an increasing curvature 

 from the point of greatest curvature to the beak, and with a short and 

 rather abrupt curvature to the front, the lateral slopes gently convex on 

 either side of the fold and then curving abruptly to the margin both later- 

 ally and anteriorly ; the mesial fold obsolete in the posterior half of the 

 shell and only moderately elevated above the lateral slopes in front ; 

 beak broadly pointed and strongly incurved beneath that of the opposite 

 valve; plications similar in form and number to those of the opposite 

 valve, those of the lateral slopes strongly arched longitudinally and some- 

 times with a slight median groove in front. Internally the cardinal pro- 

 cess is wanting; the hinge-plate is undivided but with the transversely 

 convex median portion differentiated from the lateral portions anteriorly, 

 at the apex of the beak a median septum divided above supports the in- 

 itial portion of the hinge-plate, but the connection between the septum 

 and the hinge-plate is very short and the septum itself rapidly becomes 

 lower and disappears at about one-fourth the length of the valve from the 

 beak, the lateral portions of the hinge-plate are produced anteriorly into 

 the crura. 



Surface of both valves marked by fine, more 'or less regular, concentric 

 striae and by occasional stronger lines of growth, the growth lines being 

 more conspicuous towards the anterior margin. The shell structure prob- 

 ably punctate. 



Remarks. This species is referred with a query to the genus llliyncko- 

 pora, because the punctate shell structure has not been observed. The 

 character of the hinge-plate of the brachial valve, undivided as it is to the 

 crural bases, agrees with the structure found in Khynchopora, this undi- 

 vided hinge-plate being unknown in any other of our Mississippian 

 rhynchonelloid shells. The species most closely resembles R. persinuata 

 in which the punctate structure of the shell has been clearly observed, and 

 it is possible that the two forms should really be considered as synonyms. 

 The relationships of these two species have been fully considered in the 

 discussion of R. persinuata. 



Horizon. Kinderhook and base of Burlington limestone. 



RHYNCHOPORA ? ROWLEYI n. sp. 

 Plate XXX, Figs. 18-21 



1908. Pugnax missouriensis ? Rowley, Mo. Bureau Geol. and Mines, vol. 

 8, 2nd ser., p. 81, pi. 17, figs. 60-62. 



Description. Shell of about medium size, wider than long, transversely 

 subelliptical in outline, the greatest width near the mid-length, the 



