SYRINGOTHYRIS 389 



diverge at an angle of about 110 degrees, to the antero-lateral and 

 anterior margins the surface slopes with a gently convex curvature; 

 mesial sinus originating at the beak where it is sharply defined, rounded 

 in the bottom and becoming broader and less sharply defined anteriorly, 

 produced in front in a rounded lingual extension of moderate length; 

 beak pointed, nearly erect or slightly incurved; cardinal area large, gently 

 concave, the curvature becoming a little stronger towards the beak, 

 sloping posteriorly from the hinge-line at an angle of from 60 to 65 

 degrees, the lateral margins rounding rather abruptly into the lateral 

 slopes of the valve; delthyrium triangular, higher than wide, on each 

 side of it is a vertically marked, differentiated portion of the cardinal 

 area, bounded by lines diverging from each side of the beak at an angle 

 greater than the divergence of the sides of the delthyrium, the base of the 

 entire triangular area so formed being more than double the base of the 

 delthyrium alone; each lateral slope bearing 20 or more simple, depressed, 

 radiating, more or less obscure plications which originate along the car- 

 dinal margins and grow successively smaller towards the cardinal ex- 

 tremities. Internally the hinge-teeth are supported by rather strong 

 dental plates which extend along the floor of the valve between one-third 

 and one-half its length from the beak along the sides of the sinus, the 

 divergence of the dental plates closely following the divergence of the 

 margins of the sinus ; the transverse, syrinx-bearing plate between the 

 dental lamella? bears a median longitudinal ridge externally and reaches 

 about half way from the beak to the hinge-line, the free inward projecting 

 point of the syrinx being rather broad, short and blunt ; muscular scars of 

 moderate size, with a distinct, median, longitudinal ridge. 



Brachial valve subsemicircular in outline, emarginate at the middle 

 anteriorly, the greatest convexity on the median line at or near the front, 

 the surface curving more abruptly to the cardinal margin, depressed 

 towards the cardinal extremities; mesial fold narrow and sharply defined 

 posteriorly, becoming broader and less well-defined anteriorly, gradually 

 elevated from nothing at the beak, rounded or somewhat flattened on 

 top; the beak short, scarcely projecting beyond the cardinal margin, 

 incurved ; cardinal area narrow, lying in nearly the plane of the valve ; 

 each lateral slope marked by simple, rounded, plications, alternate with 

 those of the opposite valve but much more strongly developed. Internally 

 the cardinal process is broad and flat, marked with fine vertical ribs and 

 grooves, the crural plates are strong and diverge from either side of the 

 cardinal process with the dental sockets excavated from their outer 

 surfaces; the muscular scars are not strongly defined, divided longi- 

 tudinally by a low, median ridge, which extends from the beak for two- 

 thirds the length of the valve. 



