196 THE DEVONIAN OF MISSOURI. 



Rhynchonella bialveata Weller, 1903, Pal. N. J., vol. Ill, p. 327, pi. XLII, figs. 9, 10. 

 Rhynchonella (?) bialveata Schuchert, 1913, Md. Geol. Surv., L. Dev., p. 377, pi. LXVI, 



' fig. 4. 

 Camarotoechia bialveata Dunbar, 1919, Tenn. Geol. Surv., Bull. 21, pi. 2, fig. 6. 



Description — "Shell small, triangular or triangular-ovate, sometimes compressed. 

 Valves nearly equally convex; beak of dorsal valve incurved; beak of ventral valve almost 

 straight and subangular; foramen narrow triangular, and continued to the apex of the 

 beak. Surface ornamented by from twelve to fourteen simple angular plications on each 

 valve; the two central of which, on the dorsal valve, die out a little before reaching the 

 beak, near which they are somewhat depressed, but towards the front they become 

 slightly elevated above the others, so as to form an indistinct mesial prominence. The 

 middle plication on the ventral valve is smaller than the others, and depressed near the 

 front so as to produce a faint sinus, which extends about two-thirds of the way to the 

 beak, at which point the valve is most convex. The two plications bordering the sinus 

 are larger and more prominent than those on each side of them, and become obsolete 

 before reaching the beak. A few faint imbricating lines of growth are visible near 

 the junction of the valves in front." — Hall, 1859. 



Superfamily Terebratulacea 



Family Centronellidae 

 Genus Rensselaerina Dunbar 



Rensselaerina medio plicata Dunbar 

 Plate 50, figures 25, 26 



Rensselaerina medioplicata Dunbar, 1917, Am. Jour. Sci., vol. XLIII, pp. 466, 469, pi. 



II, figs. 1-9, 12-18. 

 Rensselaerina medioplicata Dunbar, 1919, Tenn. Geol. Surv., Bull. 21, pi. 2, figs. 12, 13. 



Description — "Shell decidedly elongate oval, often nearly cylindrical, but other- 

 wise having the general form of the later Rensselaerias, though always less broad- 

 shouldered. Shell substance very finely punctate. Of the exterior surface, the posterior 

 half and the lateral slopes of the shell are entirely smooth; but anteriorly the median 

 portion of both valves bears a few simple, rounded, conspicuous plications which become 

 obsolete about half-way to the beaks. The plications are separated by well-defined 

 concave interspaces that are at least as wide as the plications themselves. There is no 

 cardinal area and the cardinal and lateral slopes are rounded. The sides of the shell 

 are not incurved as in Beachia, nor steeply angulated as is commonly the case in Rens- 

 selaeria. The minute beak of the dorsal valve is concealed by that of the ventral valve, 

 which is strongly incurved and closely appressed; the pedicle foramen is minute and 

 neither resorbed nor abraded through the umbo. 



"The ventral interior differs from that of Rensselaeria in having very small vertical 

 dental plates that do not attain the bottom of the valve, while the pedicle or rostral 

 cavity between them is very deep and narrow. The muscle scar is limited to the posterior 

 half of the shell. The diductor scars are elongate and slender. Between their more 

 deeply impressed posterior portions are embraced the narrow imprints of the adductor 

 muscles. The dorsal interior has a thick and conspicuous trangular hinge plate, sup- 

 ported by two thickened crural lamellae. This plate is highly variable. Most com- 

 monly it is thickened medially into a slight triangular elevation, the actual cardinal 

 process * * - on which are the scars of the cardinal muscles; but all grada- 



tions occur from such an elevation to a decided triangular pit * for the 



insertion of these muscles, or to the other extreme where a distinct median process 

 arises * * at the sides of which lie the muscle scars. This tendency toward 

 elevation of the center of the hinge plate is in marked contrast with Rensselaeria, 



