NO. 1229. CAMBRIAN BRACHIOPODA—WALCOTT. 671 



po.sterior portions and lio on the odjifos in tho samo piano a.s thoniaro;ins 

 of the valve.s. 



Area of the ventral valve rises from the phuu^ of the margins of the 

 valve at angles varying from 10^ to 70^; broadly subtriangular when 

 the beak is projecting, otherwise rounded at the posteiior margin 

 almost as much as the area of the dorsal valve; the latter is usually on 

 the plane of the margins of the valve and broadly rounded posteriorly; 

 both areas are striated parallel to the base and divided midway diti'er- 

 ently in each valve and in an unusual manner; in the ventral valve a 

 sharp, narrow pedicle slit cuts through the area from its base to the 

 apex; this slit opens into a cylindro-conical chamber, var3'ing in size 

 and form, that terminates in tho shell substance of the beak but does 

 not penetrate to the surface of the shell; the stride of the area pass 

 into the slit and encircle the pedicle chamber; the latter is in form 

 similar to the pedicle tube of the Siphonatrotidte, except that it is 

 closed at the outer end, and the pedicle obtained egress through the 

 narrow slit in the area; the area of the dorsal valve is divided by a 

 narrow, raised, triangular space that is bordered on either side by a 

 more or less deeply impressed groove formed b}' a narrow fold of the 

 areal lamella?; outside of this groove, and between it and the outer 

 Hexure fold of the area, occurs a depressed triangular space that has' 

 ))een considered l)y authors as the scar of tho point of attachment of 

 cardinal muscles. Sometimes the inner angle of the area of the ven- 

 tral valve adjoining the pedicle slit projects forward so as to form a 

 tooth-like knob, which in the cast is shown by a deep indentation beside 

 the cast of the pedicle chamber and between the latter and the project- 

 ing cast of the undercut beneath the area. 



Splanchnoccele' of the ventral valve contined to the posterior half, 

 while in the dorsal valve it extends forward to and in some instances 

 beyond the center; in both valves it extends back to the splanchnocoele 

 part of the area, which is bounded by the tlexure lines. Traces of a 

 median septum are shown in the dorsal valve, but no detinite septum has 

 been observed in tho ventral valve. A central median ridge of varying 

 degree of size and length often extends toward the central portion of 

 the dorsal valve; when it is large a deep, rounded groove usually occurs 

 on each side of it, on the inner slope of which the central muscle scars 

 may l)e situated. Often the median ridge is practically al)sont. 



The grooves of the main vascular sinuses begin in each valve at the 

 front margin of the area near the median lino, and in the ventral valve 

 gradually extend forward and outward toward the front half of the 

 shell, where they begin to gently curve inward, terminating toward 

 the front of the valve, the distance and curvature varying in ditferent 

 species. In the dorsal valve tho main vascular sinuses curve out more 

 rapidly, and are much less prominent; none of the secondary radial 

 canals or the peripheral vascular sinuses have been observed. The 



^ Usins; nomenclature of INIickwitz. 



