266 Tlie American Geologist. • May, ifioo 
the Burlington limestone at Louisiana and that is Prof. 
Worthen's Tcrebratuhi rowleyi which is undoubtedh' a Cen- 
tronella. 
Two specimens in the author's collection were found in 
the soft white cherts of the Lower Burlington limestone at 
Louisiana, Mo. 
Hiistedia pygmaea (nov. sp.)- 
Plate V, Figs. 25, 26, 27. Ventral, dorsal, and profile views of the 
largest specimen yet found, natural size. 
Shell ovate, gibbous, length usually greater than the 
width. Brachial valve convex with from 12 to 14 rather 
strong rounded plications. Pedicel \'alve convex with a 
short, stout beak, incurved and perforate at the end. The 
same number of plications on this valve as on the pedicel. 
Lines of growth are not noticeable. The collection contains 
a dozen specimens, some of which are hardly one-sixth as 
large as the figured specimen. From the soft white cherts 
of the Lower Burlington limestone, Louisiana, Mo. 
Retzia 7 raricosta (nov. sp.). 
Plate \, Figs. 34, 35, 36, 37. Dorsal, ventral, profile and front views 
of a medium sized specimen, natural size. 
Shell sub-circular in outline, width and length equal. 
Valves rather flat, equally convex. The brachial valve has 
from seven to nine very strong rounded plications. Pedicel 
valve has from six to eight equally strong costae. This shell 
differs from typical Retzias in some respects but as nothing 
is known of the interior of the fossil it is thought best to 
leave it under Retzia for the present. It is rather rare in 
the soft cherts of the Lower Burlington limestone at Louis- 
iana. The collection contains fi\'e specimens. 
Eumetria perstrialis (nov. sp.). 
Plate V, Figs. 28, 29, 30. Dorsal, ventral and profile views of the 
only specimen yet discovered, natural size. 
Shell elongate, subtriangular in outline, strongly marked 
by a few concentric lines of growth. Length greater than 
the width, gibbous. Beak of the pedicel \'alve, strong, short. 
