466 PAL5:0NT0L0GT OF NEW-TORK. 



PLATE ex. 

 Fig. 2. The exterior surface of 4 weathered and exfoliated Bpecimen of the left yalve. 



Geological position and locality. In the Oriskany sandstone : Albany and Scho- 

 harie counties ( New-Yorlc), and Cumberland (Maryland ). 



Avicula recticusta ( n. s.)- 



Plate CIX, Fig, 8. 



Shell slightly oblique, eubrhomboidal : hinge -line greater than the 

 greatest width of the shell below ; width equal to about once and a 

 third the height, very moderately convex. Posterior wing large, ex- 

 tending nearly as far backwards as the posterior margin of the shell. 

 Anterior wing smaller, triangular, slightly concave on the outside. 



SoRFACE marked by strong dichotomizing subequal ribs, which proceed 



principally in pairs from the umbo to the margin of the shell. Posterior 



wing with fine radiating ribs and close concentric laminse ; the anterior 



wing being marked only by the concentric striae. 



A single specimen of this species only has been observed, but its form and 

 proportions indicate its distinction from the preceding. 



Geological position and locality. In the Oriskany sandstone : Helderberg moim- 

 tains. 



Avicula gebliardl. 



Plate CX. Fia.l a, 6; and Plate CXI. Fio. 2. 

 .Avicula gehhardi : Conrad, Annual Report on the Palaeontology of New-York, 1841, p. 54. 



Bhell " suborbicular : left valve convex, with about fifteen slightly im- 

 " pressed radiating grooves, forming wide convex obsolete ribs ; ears 

 " equal, not produced. Height about five inches." 



The original specimen, from which Mr. Conrad made his description, is igured 



on Plate cxi. 



PLATE CX. 



Fig. 1 a. A cast of the left valve of a specimen. 



Fig. 1 6. A smaller specimen, in which the ribs are better preserved. 



PLATE CXI. 

 The east of the left valve of a large individual of this species. 



Geological position and locality. In the Oriskany sandstone : Albany and Scho- 

 harie coimties. 



