NO. 1395. CAMBRIAN BRACHIOPODA—WALCOTT. 291 



often submucrunate; front of the valveK rounded or slightly eniarginate in the mid- 

 dle. Ventral valve the most convex, and marked by a strong, angular mesial eleva- 

 tion, nearly one- fourth as wide on the front of the valve as the width of the shell; 

 area moderately high and nearly in the plane of the valve; cardinal borders very 

 gradually sloping from the center to the extremities of the cardinal line. Dorsal 

 valve less convex than the opposite, and marked by a subangular mesial depression, 

 corresponding to the fold of the opposite valve; area linear. Surface of the shell 

 apparently smooth, or at least so far as can be determined from either the internal 

 casts or from the matrix. 



I had some doubt regarding the positive identity of this species with that described 

 by Professor Winchell in c-onsequence of the following remarks which occur in his? 

 description: "Surface with 16 or 18 ribs visible on the casts, the strongest of which 

 limit the mesial sinus," and again, " apparently of the type of Orthis hiforta." In 

 examining a number of specimens I had not been able to detect any stria^ or ribs, 

 and felt somewhat inclined to regard it as a distinct species from that one. I have, 

 however, through the kindness of Professor Winchell, been able to make a direct 

 comparison with one of his original specimens, and should consider them as identi- 

 cal. The specimen sent me has the mesial fold (of the ventral valve) remarkably 

 strongly defined by a depressed line on each margin, the center aj^pearing tumid, 

 which I think is in part due to accident. The specimen corresponds closely with the 

 ventral valve we have figured, exc^ept in the strongly defined mesial fold. 



Ohservations. — This is the same type of Syntrophia as 8. primordl- 

 alis. It differs in its more rounded outlines, shallow, rounded sinus 

 in ventral valve, and usually a less pronounced fold on the dorsal 

 valve. Evidence of a short median septiun at the end of a very short 

 spondylium in the dorsal valve is shown l)y one cast. 



Formation cmd locality. — St. Croix sandstone, north end of Devils 

 Lake near Baraboo and Trempealeau, Wisconsin. 



SYNTROPHIA BILLINGSI, new species. 



General form transversely subquadrate; rounded; biconvex, with 

 the dorsal valve elevated on the median fold. Surface marked by a 

 few imperfectlv developed ribs on some shells, while others have only 

 concentric stria? and lines of growth. Ventral valve with a relatively 

 shallow median sinus, in which obscure, narrow ribs sometimes occur; 

 none of the specimens in the collection show the area, but from the 

 profile of the valve it must have been of moderate height, with the 

 rather sharp apex curving slightl}^ over it; a single poor cast of the 

 interior shows the outline of a spondylium much like that of S. pri- 

 niordialls; the evidence of the presence of a median septum is a dark 

 line in front of the end of the cast of the spondylium, which indicates 

 that the shell sulxstanco extends down into the limestone and that it is 

 the median septum. Dorsal valve with a small umbo and apex that 

 extends forward into a prominent median fold that may be plicated or 

 smooth; one partially exfoliated shell shows three plications on tlie 

 fold and three or four on each lateral slope of the valve. 



Oh)<ervations. — This species belongs to the plicate group of the 

 genus represented b}' S. texana and S. ahioniii.s. It differs from S. 



