BRACHIOPODA. 195 



Orthis Jamesi, wliich is placed in this association, there is occasionally a devia- 

 tion toward the resupinate contour exemplified in the Groups IV and V, 



DINORTHIS 



(num. iiropos). 



III. Group of Orthis pectinella (Conrad, MS.), Emmons. 



Pl.ATK V, FIGS. 27-36. 



1842. Orthis, EmmOxNS. Geology N. Y. ; Rept. Second Dist., p. 394, (ig. 2. 



1847. OHUs, Hall. Pala;ontolog-y of N. Y. ; vol. i, pji. 123, 124, pi. xxxii, tigs. 10, 11. 



1856. Orthis, Billings. Canadian Nat. and Geol., vol. i, p. 136, pi. ii, tig. 6, and p. 205, fig-. 5. 



IS.'iS. 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. Winchbll. Ninth Annual Rept. Geol. and N.at. 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. St.ate Geol. for 1882, pi. xxxiv, figs. 3S), 40. 



This group of shells, in its most characteristic examples, presents a reversal 

 of the relative convexity of the valves as seen in Orthis calli gramma. The ped- 

 icle-valve, elevated at the umbo, becomes gradually depressed ha growth ad- 

 vances, and in the mature condition is liat 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 costa), as in O. 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, 



