CAMBRIAN BRACHIOPODA — WAI.COTT 79 



the valve from the posterior margin ; the slope back of the foramen 

 is gently rounded and without a trace of false area or pedicle groove ; 

 the position of the beak is not clearly defined, as the margin is 

 rounded and the uniform slope of the outer surface is unbroken. 

 Dorsal valve uniformly and slightly convex ; the position of the beak 

 is indicated by a slight projection of the outline of the valve. 



Surface marked by fine concentric lines. The substance of the 

 shell is calcareous in an oolitic limestone in which semiphosphatic 

 shells of Obolns are preserved. The shell is thick and apparently 

 formed of one layer, but this is probably, as in the case of the shells 

 of Obolclla crassa (Hall) [1847, P- 290], a condition of preserva- 

 tion, the original layers or lamellae having been replaced or else 

 cemented together. The average size of the valves is from 3 to 5 mm. 



The interior of the ventral valve does not show a true area ; there 

 is a space between the margin and the end of the median furrow 

 into which the foramen opens. The median furrow is rather broad 

 and deepest at the foramen; it extends forward beyond the center 

 of the valve; the furrow into which the foramen opens is broadest 

 at the posterior end and running out to a point a little in advance of 

 the opening; from each side of the furrow and opposite the opening 

 a furrow extends obliquely outward and then forward subparallel 

 to the median furrow. Two large, oval muscle scars occur in the 

 space between the outer furrow and the postero-lateral margin of 

 the shell ; these scars correspond in position to the transmedian and 

 anterior lateral muscle scars of Obolns and Trematobohis. Nothing 

 is clearly shown of the position of the main vascular canals unless 

 the grooves outside of the median depression indicate their position, 

 or it may be that they were on the narrow ridges outside of the side 

 furrows and inside of the lateral muscle scars. 



The interior of the dorsal valve shows a rudimentary area much 

 like that of Riistella edsoni Walcott [1905a, p. 311] ; the area is a 

 smooth space with a slightly defined central depression, from which 

 a narrow, low median septum extends forward to about the center 

 of the valve; a narrow ridge extends forward from the posterior 

 central depression on each side at about the inner third of the dis- 

 tance between the median septum and the outer margin ; these 

 ridges probably marked the position of the main vascular sinuses. 

 The central muscle scars occur in the shallow depression on each 

 side of the median septum a little back of the transverse center of 

 the valve, and the transmedian scars and outside laterals are just 

 outside of the narrow ridges on each side of the valve; these scars, 

 like those in the ventral valve, are large for so small a shell. 



