<58 [ Senate 



EhYNCHONELLA NUCLEOLATA (n.S.). 



. Pal. N.Y. Vol. iii, pi. 31, f. 1 & 2. 



Shell varying from spherical to spheroid -pentagonal or sub- 

 pentagonal : ventral valve convex or depressed-convex, abruptly 

 deflected towards the margins ; beak small, depressed, closely 

 incurved over that of the opposite valve, often subangular on 

 its lateral margins : dorsal valve larger, sometimes very gib- 

 bous, often a little depressed towards the beak; beak never 

 prominent. Surface marked by fifteen to twenty-three simple 

 rounded plications, about four or five of which are slightly 

 elevated towards the front of the dorsal valve into a mesial 

 prominence, and three to five depressed on the ventral valve, so 

 as to form a more or less distinct sinus, which never extends 

 above the middle of the shell. These depressed plications are 

 prolonged in front into a more or less distinct linguiform ex- 

 tension fitting into a corresponding sinus in the front of the 

 opposite valve, and sometimes curved inwards beyond the plane 

 of a right angle with the back of the valve. 

 This species is perhaps more nearly related to Rhynchonella {Terehro' 

 tula) wilsoni, than any other species in the rocks of New- York. Indeed 

 the analogy between this one and some of the forms referred to that species 

 is so great, that until the limits of the variation to which it is subject are 

 better defined, it is scarcely possible to point out characters by which they 

 can always be distinguished. 



The species under consideration differs from authentic specimens of R. 

 wilsoni, from Dudley, England, in being uniformly more coarsely plicated, 

 and usually more angular in outline. When compared with specimens of the 

 same species from Bohemia, these differences are not so conspicuous. 



Geological position and locality. Shaly limestone of the Lower Helder- 

 berg group, Albany county < 



Rhynchonella abrupta. 



Pal. N.Y. Vol iii, pi. 31, f. 3. 



Shell transversely oval, subpentagonal : ventral valve depressed- 

 con vek, very abruptly deflected towards the opposite valve on 

 the lateral margins ; beak small, depressed on the outside and 



