Maryland Geological Survey 365 



Uncinulus nucleolatus (Hall) 

 Plate LXIV, Figs. 16, 17 



Rhynchonella nucleolata Hall, 1857 (in part), Tenth Ann. Rept. N. Y. State 



Cab. Nat. Hist, p. 68. 

 Rhynchonella nucleolata Hall, 1859 (in part), Nat. Hist. N. Y., Pal., vol. iii, 



p. 227, pi. xxxi, figs. 2f-2y (not figs, la-lc = U. globulus, Id-lf and 2a-2e 



= ? C/. globulus). 

 Uncinulus nucleolata Hall and Clarke, 1893, ibidem, vol. viii, pt. ii, p. 199. 



Description. — " Shell varying from spherical to spheroid-pentagonal 

 [this refers to the forms here taken out and named U. glohulus] 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 gibbous, often a little depressed towards the beak ; 

 beak never prominent. Surface marked by fifteen to twenty-three [usual 

 number eighteen] 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 beyond the middle of 

 the shell. These depressions are prolonged in front into a more distinct 

 linguiform extension 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." Hall, 1859. 



This species, as illustrated by Hall, embraced at least two forms, but his 

 description and the greater number of illustrations are here accepted as 

 the author intended to apply this name to Helderberg rhynchonelloids. 

 The smaller globular forms are here taken out of U. nucleolatus and dis- 

 tinguished under the name U. glohulus. This leaves the larger and more 

 ti-iangular (in section or side view) shells under the name U. nucleolatus. 

 As thus restricted, the species approaches closely to U. pyramidatus of 

 the same geological horizon, but the latter can be distinguished by their 

 fewer plications. It may prove that this difference has no greater value 

 than a varietal one, but in that event the name will be useful. This coarsely 

 plicated form does not occur in Maryland, and it is not yet established 

 that U. nucleolatus occurs in the New Scotland zone about Cumberland. 



