TUENTON PERIOD. 75 



It is too voluminous for transscription here, and the reader is referred to the 

 great works of Davidson on the Fossil Brachiopoda for the most complete 

 exhibition of it yet piiblished. The specimens contained in the collection 

 are all refen-ed to the variety generally known as OrtJds lynx Eichwald. 



Position and locality. — Lower Silurian strata of the Cincinnati epoch ; 

 Silver City, New Mexico, where it is associated with equally well-known 

 forms of that epoch. 



Family RHYNOHONELLID^. 



Genus EHYNCHONELLA Fischer, 1809. 

 Rhynchonella argenturbica Wbite. 



Plate IV, fig. 12 a, i, c, d, and c. 



Bhynchonella argenturbica White, 1874, Exp. & Surv. west 100th Merid., Prelim. Rep. 

 Invert. Poss., 14. 



Shell somewhat less than medium size, compact, subtrihedi'al in out- 

 line; length and width nearly equal; maximum height in old shells nearly 

 equal to the width; postero-lateral margins somewhat straightened or 

 slightly convex; rosti-al angle from forty to forty-five degrees; antero- 

 lateral margins rounded; front, viewed from the dorsal or ventral side, either 

 sinuous or truncate. 



Dorsal valve more convex than the ventral, abruptly arching behind 

 the middle; break strongly incurved; mesial fold very prominent, distinctly 

 defined even to the umbo, divided into either three or four prominent 

 angular or sharply-rounded plications; sides regularly arching to the 

 margins, both longitudinally and transversely, but become somewhat laterally 

 flattened near the beak; each side marked by from four to seven plications, 

 those nearest the mesial fold being of about the same size as those ujion it, 

 but they become smaller toward, and obsolete upon, the postero-lateral 

 margins. 



Ventral valve less capacious than the dorsal, and also l^s strongly 

 arched; beak prominent; mesial sinus deep, occupying about one-half the 

 width of the shell at the front margin, its sides abrupt and its bottom bear- 

 ing either two or thi'ee plications like those of the dorsal fold; sides sloping 

 away from the edges of the sinus with less convexity than the sides of the 



