PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 827 
flattish brachial valve and subdued ornamentation in which the concentric fila 
are prominent. It is not quite like any other species so far described. It differs 
from D. geniculata in the strength of the concentric elements of the ornamenta- 
tion, the gentle convexity of the pedicle valve, and the short trail. 
DACTYLOGONIA EXTENSA Cooper, new species 
Plate 210, B, figures 2-5 
Lepiaena incrassata TWENHOFEL and Wuittnc (not Hall), Geol. Soc. Amer. Special Pap. 11, 
p. 50, pl. 7, figs. 17, 18, 1938. 
Shell of about medium size for the genus, with the widest part slightly anterior 
to the hinge; lateral margins sloping inward strongly ; anterior margin broadly 
rounded; ornamentation consisting of groups of 5 to 7 fine costellae bounded 
by stronger costellae. 
Pedicle valve with posterior half of shell concave to flat in lateral profile, the 
concavity located just anterior to the umbo; flanks of posterior half slightly con- 
cave but rising to the place of geniculation about 7.5 mm. anterior to the beak. 
Angle of geniculation variable; geniculated portion long and convex, most con- 
vex at and just anterior to point of flexure; length of geniculate portion greater 
than the length of the body part of the shell. Anterior slope steep, decreasing 
posterolaterally to the cardinal extremities where it becomes considerably flat- 
tened. Palintrope unusually long, apsacline. 
Brachial valve with slightly convex body part and abruptly bent geniculated 
portion. 
Measurements in mm.—Holotype, length 13, width 19, hinge width 18, sur- 
face length 18, body part surface length 7.5, surface length of trail 10.5. 
Types.—Holotype: 117584b; figured paratype: 117584a. 
Horizon and locality—Mingan formation (lower third), Bald Island, Mingan 
Group, St. Lawrence River. 
Discussion.—This species is characterized by its long trail and the rounded 
area of geniculation. It is most like some extreme examples of D. incrassata but 
the angle of geniculation is greater and the trail longer. The Mingan form does 
not have the geniculated part so narrowly rounded even though tangents to the 
visceral region and the trail intersect in an acute angle. The angle is about 75° 
in D. extensa but only 55° in D. incrassata, The interarea of D. extensa is much 
longer and the pseudodeltidium larger than in D. incrassata which has only a 
short interarea. 
DACTYLOGONIA GENICULATA Ulrich and Cooper 
Plate 216, D, figures 14-30; plate 217, M, figures 38, 39; plate 218, B, figures 4-7; 
plate 219, E, figure 11; plate 219, F, figures 12, 13 
Dactylogonia geniculata ULRicH and Cooper, Journ. Paleont., vol. 16, No. 5, p. 624, pl. 90, 
figs. 19-23, 1942. 
Shell wider than long with acute cardinal extremities ; profile concavo-convex ; 
lateral margins sloping medially ; anterior margin broadly convex to subtruncate. 
