288 The American Geologist. May, 1892 
apically perforated deltidium, which is somewhat wider than 
long, and excavated posteriorly for the reception of that of the 
other valve. 
Brachial (dorsal) valve nearly flat or slightly convex, with a 
shallow mesial sinus, having its origin near the beak and rapidly 
widening to the anterior margin, which is more or less sinuous 
according to the depth of the medial depression. Cardinal area 
linear with a short, broad deltidium partially covering the cardinal 
process. 
This little species of Strophomena is separated readily from all 
other biconvex forms of this genus, in having a rapidly widening 
mesial sinus in the brachial (dorsal) valve, and the pedicle valve 
sub-angular along the median line, while in all other related spe- 
cies these characters are the reverse. 
Plate xxx1, figs. 21-24. 
Formation and locality. Near the base of the Galena, associated. with 
Clitambonites diversa, south of Cannon Falls. 
Leptzna charlotte. 
Shell small, transversely semi-oval, plano-convex, geniculate, 
with the sides slightly convex, and converging to the broadly 
rounded front, or drawn out tongue-shaped; hinge-line as long 
or somewhat shorter than the greatest width of the shell.  Sur- 
face marked by tine, closely crowded, alternating strive as in 
Rafinesquing alternata, crossed by exceedingly delicate concentric 
lines, and over the visceral disc of each valve by more or less 
continuous zigzag undulations or wrinkles, 
L. charlotte ditters from any other American species of Lep- 
fend in its zigzag concentric surface corrugations, and other 
minor features which can be seen more readily in the illustrations 
than by any written comparison, Plate xxxu1, figs. 1-5. 
Plate xxx11, figs. 1-5. 
Formation and locality. Upper portion of the Trenton limestone and 
base of the Trenton shales in the Bryozoa layers at Minneapolis and 
St. Paul. 
Plectambonites gibbosa. 
Shell small, semi-circular in outline, strongly concayvo-conyex, 
wider than long, greatest width along the hinge-line. Surface very 
finely striate, with six or seven stronger striations on each valve, 
muchas in 2. transversalis. 
Pedicle (ventral valve) very gibbous and sub-carinate medially ; 
