UTICA SLATE AND HUDSON=RIVER GROUP: 291 
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ACEPHALA OF THE UTICA SLATE AND HUDSON-RIVER GROUP. 
Pruates LXXX., LXXXI. & LXXXII. 
Among the AcepHALA, not more than three species are known in the Utica slate; and 
these, with one exception, are continued into the higher shales. In the shales, and 
argillaceous and calcareous sandstones succeeding the black slate, we find a considerable 
accession of species, which are unknown in the limestone below. At the same time, several 
species are common to the limestones and the succeeding shales and sandstones. The 
Genus Avicuta, of which the species in the limestone are somewhat equivocal, becomes 
well characterized in two species. The Genus AMponycuia is represented by two species 
quite distinct from those in the lower strata; and the Genus Mopio.opsis presents very 
characteristic forms, which are much more numerously developed than in the inferior 
limestone. In addition to these, there are two or three other forms, presenting generic 
characters not observed in those previously described. Two of these ( OrTHoNnoTa and 
CLerpopHorus ) become more distinctly developed in succeeding formations. 
The comparative abundance of species and individuals of the Bracuiopopa and 
AcEPHALA in these strata may be readily appreciated by reference to the plates (79, 80, 
81 and 82), where the species of each are represented. 
346. 3. AVICULA INSUETA. 
Pu. LXXX. Figs. 1 a, b. 
Avicula insueta. Conrap in MS. 
—_ — Emmons, Geol. Report, 1842, pag. 399, fig. 5. 
Shell obliquely subrhomboidal, depressed convex ; hinge line extended ; anterior wing 
short, obtuse or rounded ; posterior wing triangular, acute, extending a little beyond the 
margin of the shell; surface marked by unequal concentric strie and stronger wrinkles, 
and longitudinally, along the middle of the shell, by obscure radii. 
This fossil bears some resemblance to other species of the genus in the higher strata, but 
differs essentially on a-careful comparison. 
Fig. 1 a. A specimen, natural size. 1. A portion of the surface enlarged. 
Position and locality. This species occurs in the lower black shale, or Utica slate, at 
Canajoharie, and is not known in the higher part of the group. (State Collection ) 
37* 
