PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 997 
Measurements in mm.— 
Brachial Hinge 
Length length Width width Thickness 
BOS BSCE aus beth tests toieidatohs 7.5 6.9 7.7 6.0 4.9 
CHOROR Ware ticki clas eiedeisi aie ses yey 6.5 7.7 6.3 4.9 
Figured specimens.—86583a-c ; measured specimen: 86583d. 
Horizon and locality —Chaumont formation in New York: Loose in the bed 
of Black River just below the third dam at head of island, Watertown, Water- 
town (15’) Quadrangle. 
Discussion.—As noted above, this species is very similar, if not identical, to 
P, sulcata. 
DIORTHELASMA Cooper, new genus 
(Greek di, two; ortho, straight; elasma, plate) 
Shell minute, subrectangular in outline; sides rounded; anterior commissure 
strongly sulcate; unequally biconvex, the pedicle valve much deeper than the 
brachial valve. Surface somewhat distantly costellate. Punctate? 
Pedicle valve with small teeth, curved and apsacline interarea, and: small, in- 
curved beak. Delthyrium partially closed by lateral plates. 
Dental plates receding, short, divergent. Delthyrial cavity shallow; muscle 
field wide and short, with large diductor scars and short, narrow adductors; 
floor of muscle field thickened. Vascula media strongly divergent when visible. 
Brachial valve with shallow and narrow notothyrial cavity bounded by slightly 
divergent or slightly convergent, erect brachiophore supporting plates; brachio- 
phores long and slender; fulcral plates rudimentary; cardinal process varying 
from a slender ridge to a thick triangular boss filling the notothyrial cavity. 
Adductor scars small, elongate, the posterior pair located outside the anterior 
pair ; no median ridge. 
Genotype.—Diorthelasma parvum Cooper, new species. 
Discussion—This genus can be briefly characterized as having an interior like 
that of Pionodema or Fascifera, in combination with very unequal valves, the 
pedicle one being deep and the brachial valve nearly flat. 
Structurally this genus is allied to Pionodema and is the earliest known mem- 
ber of the Schizophoriidae. The pedicle valve is deep and has a strongly swollen 
profile. The dental plates are short and somewhat bowed and surround a wide 
and short muscle field. No apical plate was seen in any of the specimens studied. 
Inside the brachial valve the cardinalia are very close to those of Pionodema 
but are often variable between specimens. In some instances the plates are fairly 
closely crowded, in others they are fairly distant. The cardinal process is like- 
wise variable. In some specimens it is a thin ridge, but in others it forms a 
thick, triangular boss almost completely filling the notothyrial cavity. Up to the 
present writing this genus has been found only in the Pratt Ferry formation, 
Alabama, where it is fairly common. 
