704 SYSTEMATIC PALEONTOLOGY 



Leptosolen biplicata Gabb, 1876, Proc. Acad. Nat. Sci., Phila., p. 304. 

 Leptosolen biplicata Whitfield, 1885, Mon. U. S. Geol. Survey, vol. ix, p. 



183, pi. xxv, figs. 1, 2. 



Leptosolen biplicata Johnson, 1905, Proc. Acad. Nat. Sci., Phila., p. 17. 

 Leptosolen biplicata Weller, 1907, Geol. Survey of New Jersey, Pal., vol. iv, 



p. 624, pi. Ixx, figs. 30, 31. 



Description. " Thin, convex, with two radiating folds or depressions 

 anteriorly ; basal line slightly contracted or incurved ; anterior side short ; 

 extremity truncated ; posterior margin obtusely rounded, posterior side 

 concentrically lineated ; valves somewhat contracted obliquely from beak 

 to base." Conrad, 1858. 



Type Locality. Owl Creek, Tippah County, Mississippi. 



Shell very thin, porcellanous, compressed, rudely cylindrical in outline ; 

 dorsal and ventral margins parallel, the posterior symmetrically arcuate, 

 the anterior rounding, somewhat obliquely, into the base; lunule and 

 escutcheon not defined ; umbones very inconspicuous, scarcely rising above 

 the dorsal margin, set back from the anterior extremity a distance of 

 approximately one-fourth the total latitude ; posterior area differentiated 

 by the abrupt strengthening of the concentric sculpture along a line 

 extending from the umbones to the posterior extremity of the basal mar- 

 gin, concentric sculpture reduced to faint and rather irregular incremental 

 striations upon the anterior and medial portions of the shell, least feeble 

 medially and appearing upon the posterior area as sharp-edged, regularly 

 overlapping concentric laminae ; radial sculpture not developed ; ligament 

 marginal, opisthodetic, seated upon a nymph about one-eighth as long as 

 the posterior dorsal margin ; a single very prominent subumbonal cardinal 

 in each valve ; shell reinforced within by a rather heavy deposit of calcite 

 along a vertical dropped from the umbones, the ridge thus formed broadest 

 and most elevated dorsally and gradually evanescing toward the base ; 

 muscle scars subequal, inconspicuous ; pallial sinus profound. 



Casts of the interior are remarkable for their cylindrical outline and for 

 the deep sulcus formed by the internal rib, which cuts across the umbone 

 and persists'a little more than half-way down to the ventral margin. 



