BRACHIOPODA. 411 



Leptaena unlcostatu.] 



dental plates, are the distinct scars of the small adjusters. Surface marked by deli- 

 cate, crowded papillae, strongest in front of the muscular area, and in the thin shells 

 by the wrinkling of the outer surface. 



Dorsal valve nearly flat, with the anterior margin more or less reflexed down- 

 ward. Cardinal area narrow, about one-third that of the other valve, with a broad 

 and strongly convex chilidium. Dental sockets deep; crural plates slender, very 

 oblique and merging into the median thickening, upon which is situated the small, 

 bilobed. cordate cardinal process; in front of this is a short, low septum separating 

 the small adductor scars; in the central portion of the valve there are three other 

 inconspicuous septa. Just inside the outer margin of the valve is situated a prom- 

 inent, rounded ridge of the same nature as that in L. rhomboidalis. 



L. charlottce differs from any other American species of Leptcena in its zigzag, 

 concentric, surface corrugations and in other minor features, which can be more 

 readily seen in the illustration than by written comparisons. 



Formation and locality. This species, in a dwarfed condition, is first met with in the upper portion 

 of the Trenton limestone, and is not uncommon in the upper part of the middle third of the Trenton 

 shales in the Bryozoa layers at Minneapolis and St. Paul,"Minnesota. 



Collectors. E. O. Dlrich and the writers. 

 Mm. Reg. No. 8142. 



LEPTAENA UNICOSTATA Meek and Worthen, sp. 



PLATE XXXII. FIGS. 6-9. 



1868. Strophomena unicostata MEEK and WOKTHEN. Geological Survey of Illinois, vol. iii, p. 335 



pi. iv, flg. 11. 

 1882. Strophomena unicostata WHITFIELD. Geology of Wisconsin, vol. iv, p. 262, pi. XII, flg. 14. 



Original description: "Shell transversely subsemicircular, the greatest breadth 

 being on the hinge margin, which terminates in rather acutely angular extremities; 

 lateral margins generally nearly straight, or more or less concave in outline and 

 converging from the extremities of the hinge to the front, which is rounded, a little 

 straightened or slightly sinuous in the middle; geniculation of both valves from the 

 ventral side, very abrupt all around the anterior and lateral margins to near the 

 extremities of the hinge. Ventral valve almost perfectly flat [or slightly convex for 

 a short distance anterior to the beak] and without any traces of concentric wrinkling 

 on the disc between the hinge and geniculated front and lateral margins [in large 

 Minnesota specimens there is more or less concentric wrinkling present in both 

 valves, strongest along the cardinal line on each side of the beak and nearly obsolete 

 medially] ; beak very small or scarcely distinct from the cardinal margin and show- 

 ing the usual minute [pedicle] perforation; area narrow, but a little wider than that 

 of the other valve, and slightly arched and provided with a rather wider triangular 

 fissure, closed by the convex deltidium [chilidium] and the cardinal process of the 



