tTICA SLATE AND HUDSON'-BrVER GROUP. 291 



ACEPHALA OF THE UTICA SLATE AND HUDSON-RIVER GROUP. 

 Plates LXXX., LXXXI. & LXXXU. 



Among the Acephala, not more than tliree 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 tho black slate, we find a considerable 

 accession of species, which are unknown in the limestone below. At tlie 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 INIodiolopsis 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 

 Cleidophords ) 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. I a,b. 



Avicula insueta. Conrad in MS. 



— — Emmons, Geol. Report, 1S42, 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 imequal 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, 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 tiie lower black shale, or Utica slate, at 

 Canajoharie, and is not known in the higher part of the group. (State Collection ) 



37' 



