PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 617 
being longer and more elevated. The sulcus of the Lebanon species is short and 
deep. The Tennessee species have a tendency to some costation on the front of 
the valves, which is seen only indistinctly in the Ontario form. 
DIDYMELASMA LONGICRURUM Cooper, new species 
Plate 120, D, figures 19-29 
Shell small, syntrophoid in outline and profile, subcircular; sides rounded; 
anterior margin truncated; anterior commissure somewhat narrowly uniplicate ; 
surface smooth but many specimens with indistinct costae in adults, usually 2 
on the fold, I in the sulcus, I on the flanks near the middle. 
Pedicle valve fairly strongly convex in lateral profile ; broadly and moderately 
convex in anterior profile. Umbonal region somewhat narrowed at the beak; 
median region inflated; sulcus originating just anterior to the middle, abruptly 
deepening to become moderately deep at the margin; sulcus bounded by nar- 
rowly rounded costae; flanks somewhat swollen and with steep lateral slopes. 
Interior with small spondylium and long, thin median septum. 
Brachial valve moderately convex in lateral and anterior profiles and with a 
depth about equal to that of the pedicle valve. Umbo narrow; median region 
swollen ; fold narrow and rounded, moderately strongly elevated, originating at 
about the middle; sides of fold defined by moderately deep sulci. Flanks only 
gently inflated and with moderate lateral slopes. Interior with 2 parallel brachial 
plates and long, nearly straight brachial process. 
Measurements in mm.— 
Brachial Hinge 
Length length Width width Thickness 
Eloltyper sot aweedy. Mee 375 ay 4.2 0.5 2.3 
Paratype {(rr7nssd) ais. FOske 3.5 3.4 3.75 0.49 1.3 
Types.—Holotype: 117158c; figured paratypes: 117158a,b,d; unfigured para- 
types: 117158¢,f. 
Horizon and locality—Upper part of the Lebanon formation with Doleroides 
tennesseensis Cooper in Rutherford County, Tennessee: On U. S. Highway 41, 
3 mile southeast of Knox Branch, 9 miles southeast of Murfreesboro. 
Discussion.—This species is characterized by its small size and syntrophoid 
form. It has a deep sulcus on the pedicle valve and a strong, elevated fold on 
the opposite valve. These latter features are more pronounced than in D. ab- 
ruptum which has a broader fold and sulcus. 
Superfamily RHYNCHONELLACEA Schuchert, 1896 
Family PROTORHYNCHIIDAE Schuchert, 1896 
Rhynchonellacea lacking complete deltidial plates but have dental lamellae, long 
curved crura, but no median septum or cruralium. 
Subfamily PrororHyncuiinar Schuchert, 1806 
Protorhynchiidae of uncertain structure. 
