UTICA SLATE AND HUDSON-RIVER GROUP, 
291 
ACEPHALA OF THE UTICA SLATE AND HUDSON RIVER GROUP. 
Plates 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 Avicula, of which the species in the limestone are somewhat equivocal, becomes 
well characterized in two species. The Genus Ambonychia is represented by two species 
quite distinct from those in the lower strata ; and the Genus Modiolopsis 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 ( Orthonota and 
Cleidophorus ) become more distinctly developed in succeeding formations. 
The comparative abundance of species and individuals of the Brachiopoda 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. 
Pl. LXXX. Figs. 1 a, b . 
Avicula insueta. Conrad 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 striae 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, hut 
differs essentially on a careful comparison. 
Fig. 1 a. A specimen, natural size. 1 b. 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* 
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