BRACHIOPODA. 
195 
Orthis Jamesi, which is placed in this association, there is occasionally a devia¬ 
tion toward the resupinate contour exemplified in the Groups IV and V. 
DIN ORTHIS 
(nom. propos). 
III. Group of Orthis pectinella (Conrad, MS.), Emmons. 
PLATE V, FIGS. 27-36. 
1842. Orthis, Emmons. Geology N. Y.; Rept. Second Dist., p. 394, fig. 2. 
1847. Orthis, Hall. Palaeontology of N. Y.; vol. i, pp. 123, 124, pi. xxxii, figs. 10, 11. 
1856. Orthis, Billings. Canadian Nat. and Geol., vol. i, p. 136, pi. ii, fig. 6, and p. 205, fig. 5. 
1858. Orthis, Rogers. Geology of Pennsylvania, vol. ii, p. 818, fig. 602. 
1863. Orthis, Billings. Geology of Canada, p. 165, fig. 147. 
1881. Orthis, N. H. Winchell. Ninth Annual Rept. Geol. and Nat. Hist. Surv. Minnesota, p. 117. 
1882. Orthis, Whitfield. Geology of Wisconsin, vol. iv, p. 259, pi. xii, fig. 8. 
1883. Orthis, Hall. Rept. N. Y. State Geol. for 1882, pi. xxxiv, figs. 39, 40. 
This group of shells, in its most characteristic exatnples, presents a reversal 
of the relative convexity of the valves as seen in Orthis calligramma.. The ped¬ 
icle-valve, elevated at the umbo, becomes gradually depressed as growth ad¬ 
vances, and in the mature condition is flat or gently concave over the pallial 
region. The brachial valve, on the other hand, is eminently convex. The 
surface is marked by strong, simple, rarely bifurcating costae, as in 0 . calli¬ 
gramma. The cardinal area of the pedicle-valve is well developed but not 
greatly elevated. In the interior the dental lamellae -are prominently devel¬ 
oped and are extended around a subquadrate muscular area, the strength of 
which apparently depends upon the age and thickness of the shell. The three 
pairs of impressions may often be distinguished; the elongate adductors 
occupying a central position and separated by a faint median ridge, the diductors 
forming large ante-lateral expansions enclosing the adductors; the adjustors 
lie outside and behind these. Occasionally, in Orthis pectinella, there is again 
seen the gradual closing of the delthyrium of the pedicle-valve by an apical 
callosity, but it is never carried as far as in the forms mentioned in the group 
of Orthis calladis, and, so far as observed, its existence is confined to the 
species cited. In the brachial or more convex valve the area is narrower, the 
crural plates stronger than in the preceding groups, and the cardinal process, 
instead of being a simple linear ridge lying in the bottom of the deltidial cavity, 
