252 NEW YORK STATE MUSEUM 



byssal margin 4 mm; a small shell, hight 3.5 mm, length 4.5 mm, length 

 of byssal margin 2 mm; a large example, hight 8 mm, length 10 mm, 

 length of byssal margin 4 mm. 



Habitat. Genesee province ; Naples subprovince. Widely distrib- 

 uted. More commonly found in the bituminous shale beds near the base 

 of the group than in the olive shales and sands. Havana glen, Montour 

 Falls, Schuyler co., Glenora, Rock Stream, Belknap's gully near Branchport, 

 and south of Perm Yan, Yates co. ; Parrish gully and elsewhere, Naples ; 

 at various spots in the towns of South Bristol and Bristol, Ontario co. ; on 

 the shores of Canandaigua and Conesus lakes, etc. ; in the Cashaqua or 

 lower shales at Griswold, Genesee co., Eighteen Mile and Pike's creek, 

 Erie co. Chautauqua subprovince. Rare on Farnham creek near 

 Angola, 250 feet above the Cashaqua shales. In the Styliola limestone at 

 many outcrops. 



In the Marcellus and Hamilton shales, and the dark shales of the 

 Genesee common at various localities. 



Pterochaenia fragilis Hall (sp.) 

 var. orbicularis var. nov. 



Plate 4, fig. 17, 18; plate 5, fig. ii-z6 



The attenuate, narrow, erect and prosogyre beak is shown in this 

 variety as in the specific type. The outline is suborbicular, varying to 

 subquadrate, and the hiatus is relatively short ; the byssal margin being 

 gently incurved. Hight and length equal. Surface as in P. fragilis. 



The specimens of this variety in the Naples fauna are usually of small 

 size, measuring approximately 5 mm in hight and length. In some locali- 

 ties twice this size is attained. 



Habitat. Genesee province ; Naples subprovince. At Naples and in 

 the vicinity; also near Ithaca (Cayuga lake inlet) and in the Ithaca faun-a at 

 Norwich, Chenango co. and in the Genesee shales on Cayuga creek, at Iron 

 Bridge Mills. The larger form of this variety, like that occurring in the 

 locality last mentioned, is also found in the dark shales of the Hamilton 

 group south of Aurora N. Y. 



