SYRINGOTHYRIS 387 



front to beak, nearly straight or slightly convex from the cardinal to the 

 front margins towards the central portion of the valve, becoming a little 

 concave towards the cardinal extremities because of the slight vertical 

 compression of the valve along the cardinal margin, the anterior slope 

 along the median line nearly straight or slightly curved; mesial sinus 

 narrow and rather sharply defined at the beak, becoming rapidly broader 

 towards the front and much less well-defined, non-plicate, concave in the 

 bottom and produced anteriorly in a broad, rounded, linguiform extension 

 of moderate length ; beak pointed, erect, rarely with a slight incurvature ; 

 cardinal area high, sloping posteriorly from the hinge-line at an angle 

 of from 25 to 30 degrees to the plane of the valve, broadly triangular, 

 the lateral margins sharply defined, the surface flat or very slightly 

 convex, rarely with a very slight concavity just beneath the beak; 

 delthyrium narrowly triangular, on each side of the delthyrium is a 

 differentiated region bounded by lines which originate at the beak and 

 diverge towards the hinge-line at a greater angle than the divergence of 

 the margins of the delthyrium, the base of this broader triangle being 

 nearly twice the width of the delthyrium at its base, the surface of this 

 differentiated region is vertically marked while the remaining portion of 

 the area is marked by transverse lines of growth; each lateral slope 

 bearing about eighteen simple, depressed, radiating plications, which 

 originate along the cardinal margin and become gradually finer and more 

 obscure towards the cardinal extremities. Internally the dental plates 

 are rather thick and widely diverging, and extend nearly or quite half 

 way from the beak to the anterior margin of the valve on either side of 

 the mesial sinus ; the transverse, syrinx-bearing plate between the dental 

 lamellae extends between one-half and one-third the distance from the 

 beak to the hinge-line, it is usually transversely concave externally or 

 rarely with a slight median ridge towards its lower extremity, the free 

 extension of the syrinx towards the hinge-line is very short, being no 

 more than a median angular point from the margin of the transverse 

 plate, the internal casts exhibit the characteristic split tube upon the 

 inner surface of the transverse plate; muscular scars rather large, 

 subovate in outline, not sharply defined on any of the specimens observed. 

 Brachial valve broadly subtriangular in outline, with the apex of the 

 triangle in front truncated and emarginate, the surface convex through- 

 out from the cardinal to the anterior margins, the curvature more abrupt 

 to the cardinal margin, the greatest convexity at or near the anterior 

 margin at the median line of the valve, the surface sloping from the 

 median line to the cardinal extremities in a gently convex curve ; mesial 

 fold non-plicate, well-defined, narrow posteriorly and becoming rapidly 

 broader towards the front ; the beak short, scarcely extended beyond the 

 hinge-line ; cardinal area linear ; each lateral slope marked by simple 



