282 
PALAEONTOLOGY OF NEW-YORK. 
ACEPHALA OF THE NIAGARA GROUP. 
Fossils of this class are comparatively rare throughout the group ; the single species, Avicula 
emacerata , being almost the only one which is common. In these fossils the Clinton group is 
much ihore prolific, as shown by the forms figured on Plates 27 and 30. We must, therefore, 
necessarily attribute much influence to the general conditions of the period; for we find in the 
less calcareous, and more arenaceous shales and impure sandstones of the Clinton group, a 
much greater development of the acephalous bivalves than in the succeeding period, which, in 
most other palaeozoic forms, is far more prolific. The same condition in regard to these forms 
prevails through the succeeding rocks of the Silurian period ; and it is not until we reach the 
Hamilton group, that we find them as freely developed, and as numerous as in the higher 
portions of the lower silurian strata. 
472. 6. AYICULA EMACERATA. 
Pl. LIX. Fig. 1 a - e. 
Avicula emacerata. Conrad, Jour. Acad. Nat. Science, Yol. viii, 1842, pag. 241, pl. 12, fig. 15. 
— — Hall, Geol. Rep. 4th Dist. N. York, 1843, p. 109, and fig. 4, 4 a, p. 108. 
— — Id. page 83 of this volume. 
Obliquely somewhat obovate, plano-convex, the right valve flat, nearly smooth, or with fine 
concentric striae, and a few radii on the wing; left valve convex, and marked by strong ra¬ 
diating striae, which are decussated by less conspicuous concentric striae; hinge-line straight; 
posterior wing extended nearly or quite as far as the posterior margin of the shell, and ter¬ 
minating in an acute point, between which and the body of the shell the wing is more or less 
deeply arcuate ; anterior wing short, triangular, with the extremity obtuse or rounded ; umbo 
often elevated a little above the hinge-line. 
This species is readily recognized by its left valve, the strong rays of which are regularly 
cancellated by concentric striae. The right valve is rarely seen, and it appears to have been 
extremely thin and fragile, nearly or quite flat, marked on the body of the shell by concentric 
lines only, while the wing has sometimes a few obsolete radiating striae. In consequence of the 
depressed form of this valve, the line of separation between the wing and the body of the shell 
is not distinctly marked. In the convex valve the extent of the posterior wing is variable, and 
the anterior wing is sometimes continued in the direction of the hinge-line, and sometimes curved 
downwards, and the extremity considerably below the cardinal line. All the specimens seen in 
this group occur in the soft shale, and they are consequently more or less compressed. 
Fragments or single valves of this species are not uncommon ; and in a few instances, slabs 
of shale have been seen entirely covered by the separated valves. 
