vertical furrow, commencing at the ventral margin. Marginal ridge.moderately prominent, 
and along the ventral margin it is flattened and form3 a longitudinal area, widest behind. 
The length is less than a millimetre, yet the shell is so abundant that a four foot bed of 
limestone is made up ofthem in the St. Louis Limestone atPella, Marion county. 
Beyrichia llthofactor var. velata. Associated with the preceding is a variety with faint 
longitudinal ridges on each valve, which give them the appearance of being veiled. 
GENUS MEEKELLA. (n. c„) 
The genus Hemipronites^ Tuncier^ 8treptorhynchus, King—has hitherto been made to 
include a group of plicated shells, of which Plicatula Striato costata , Cox= Orthisina 
Missouriensis , Swallow—may betaken as the type. 
Messrs. Meek and Hayden in their Palaeontology of the Upper Missouri, refer distinctly 
to this group, and suggest that it may possess full generic characters, which their material 
did not clearly show. Our collections, however, show characters which clearly separate this 
group from any other known genus, for which we propose the generic name of Meekella , in 
honor of that eminent palaeontologist, Mr. P. B. Meek. The characters refered to have been 
principally observed upon the ventral valve. Instead of the linear median septum of the 
ventral valve of Hemipronites which separates the muscular impressions, and extends from 
beneath the beak to the centre of the valve, our shell has no median septum. Instead of 
having the dental processes, converging toward the beakbeneath the area, and not supported 
from the bottom of the valve by dental lamelhe as 'in that genus, the ventral valve of our 
shell has two broad dental lamella:, which are continuous from the cardinal tee th to the beak, 
passing directly in front of the sutures between the cardinal area and the pseudo-deltidium, 
thence slightly diverging, they extend forward along the bottom of the shell toward the front, 
the anterior margins of the lamallse arching backward and u p ward to the dental processes.— 
Thus a cross section anywhere in the rear of the hinge line, would show the outline of three 
rostral chambers. 
The cardinal process of the dorsal valve is large, curving backward in front of the pseudo- 
deltidium, and has upon each side of it a wing-like expansion which are curved up at their 
outer edges to form fosettes for the dental processes of the opposite valve. 
We have seen some indication of a coarsely punctate structure, in the typical species, but 
we are not yet satisfied upon this point. 
The broad dental lamellae with the three rostral chambers ; the plicated shell and usually 
rebust form, clearly separate this genus from Hemipronites. The rudimentary or obsolete 
area of the dorsal valve, and non-perforate and usually distorted ventral beak separate it from 
Klitambonites , Pander= Orthisina , King. 
The discovery of the generic characters just described, does not, in our opinion, necessa¬ 
rily remove the group from among the Orthidce, as that family is at present recognized. Its 
affinities seem to he on the one hand with Hemipromtes , of Pander, and on the other with 
Syntrielasma of Meekand Wor then, bearing much the same relation to the former, that the 
latter does to Orthis. 
Our genus is understood to include Orthisina Shumardiana , Swallow— Productus, 
eximius , Eichwald— Sir eptorhynchus occi&entaMs andS. pyramidalis, Newberry, besides the 
typical species before mentioned. 
Iowa Citt, Iowa, May 8th, 1867. 
