532 THE PALEONTOLOGY OF MINNESOTA. 
[Prolobella. 
Genus PROLOBELLA, n. gen. 
Shell equivalved, moderately convex, very inequilateral, obliquely acuminate- 
ovate. Anterior end very small, auriculate or subnasute, sharply distinguished from — 
the body of the shell. Basal and posterior margins rounded. Hinge thin, apparently 
edentulous, rather short and not produced at the posterior extremity; just in front 
of the beaks a short clavicle-like process produces a sharp linear depression in casts. 
of the interior. Surface marked with concentric lines of growth and radial striz or 
plications. Anterior adductor scar small, situated in the anterior lobe. Posterior 
impression and pallial line not observed. 
Type: Prolobella striatula, n. sp. 
It is almost certain that Conrad’s Avicula trentonensis and aviformis, which Hall 
in 1847 united as one species, are congeneric with the Minnesota species which is 
made the type of this new genus. These shells are not true Aviculide, their valves 
being equal and without the prolonged posterior wing. Nor do they fit much better 
into any of the other families. They seem to be remnants of one of those complex 
primitive types that give the systematist so much trouble to classify. In this case 
there is almost as much reason for placing the genus with the Ambonychiude as 
with the Aviculide or the Modiolopside. With such types it is good policy to defer 
conclusions until the collector has furnished us with the missing links. And they 
will be found sooner or later, for the lower paleozoic rocks are teeming with undis- 
covered fossils. 
PROLOBELLA STRIATULA, 2. Sp. 
PLATE XXXYV, FIG. 27. | 
Shell rather small, obliquely subovate, moderately convex. Anterior end very 
small, somewhat auriculate, narrowly rounded, and rather sharply distinguished 
from the rest of the shell. Cardinal margin straight, about half as long as the 
shell posterior to the beaks, passing with a gentle curve into the posterior margin, 
which is oblique and moderately convex to the lower third where the outline bends 
rapidly forward; basal margin almost uniformly convex; anterior outline strongly 
sinuate beneath the ear. Beaks full, slightly prominent, not much incurved, just 
in front of them the cast shows a vertical linear depression extending from the 
hinge half across the narrow sinuate part of the shell. Umbonal ridge inconspicu- 
ous. Surface with a small number of obscure concentric lines, and in the antero- 
basal third with numerous very fine thread-like radiating strie. 
This species cannot be confounded with any other Minnesota bivalve known. 
From P. trentonensis Conrad, sp. of the Trenton of New York, it is readily distin- 
