51 
Locality and Formation. Several brachial valves were 
found in the Chazy limestone at MacLaren landing. 
Rafinesquina rugosa n. sp. 
Plate III, figures 8, 9 
The general shape is concavo-convex. The greatest width 
of the shell is along the straight hinge line. The width averages 
10*1 mm.; the length 7-1 mm. The sides and cardinal area 
make an angle of 90 degrees; the sides are almost straight for 
two-thirds of the length. The anterior margin is broadly 
rounded. The striations increase by interpolation and bifurca- 
tion, several fine striae lying between the coarser ones. Con- 
centric growth lines are present. The surface is rugose, with 
alternating elevations and pits, the elevations being arranged in 
concentric series greatly exaggerating the effect of the con- 
centric growth lines, particularly on the anterior margin, and on 
the wings of the hinge line. 
The pedicle valve is rounded with an even rising on the 
umbonal region, which terminates in a small but acute beak 
projecting beyond the hinge line. In the anterior third the 
convexity of the shell is decidedly increased, and a slight, 
narrow fold rises into prominence, topped by the largest median 
striation which is not particularly conspicuous in the umbonal 
region. The rugose surface ornamentation is somewhat more 
conspicuous on the pedicle valve, on account of its greater 
convexity. 
The brachial valve is slightly concave at the beaks becoming 
convex along the last quarter towards the anterior margin. The 
sinus is very slight, not being present in a great many of the 
specimens. The ornamentation is the same as in the pedicle 
valve but less conspicuous. 
It is readily distinguished from the other Trenton Rafines- 
quina by its smaller size. It is, perhaps, most nearly related to 
Rafinesquina declivis James, from which, however, it differs in 
its smaller size and straighter sides without any tendency to the 
triangular form; and in its unique ornamentation. In the latter 
respect it shows an affinity with Leptaena charlotta, from which 
it is easily distinguished by its general shape and proportions. 
