282 



PALAEONTOLOGY OF NEW-YORK. 



ACEPHALA OF THE NIAGARA GROUP. 



Fossils of this class are comparatively rare throughout the group ; the single species, Avizula 

 einacerata, being almost the only one which is common. In these fossils the Clinton group is 

 much more 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 paheozoic 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 sUata. 



472. 6. AVICULA EMACERATA. 



Pl. LIX. Fig. 1 a-e. 



ylvicida emacerata. Conrad, Jour. Acad. Nat. Science, Vol. viii, 1812, 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 somew-hat obovate, plano-convex, the right valve flat, nearly smooth, or with fine 

 concentric stria;, 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 j hinge-line straight ; 

 posterior wing extended nearly or quite as far as the posterior margin of the shell, and ter- 

 minating iji 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 striiC. 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 stri;p. 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. 



