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 striae or 

 plications. Anterior adductor scar small, situated in the anterior lo.be. 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 Aviculldce, 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 AmbonycMidce as 

 with the Aviculidce or the Modiolopsidce. 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, n. sp. 



PLATE XXXV. 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 striae. 



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- 



