NUCULID^. 251 



Distr. — Silurian ; New York, Europe. N. ovatus, Sowb. 

 (cxxvii, 62). 



Shell thin, margins not crenulated, ligament external ; no 

 cartilage-pit ; teeth numerous, on a nearly straight cardinal line ; 

 anterior muscular scar bounded by a ridge ; pallial line simple. 



Cleidophorus, Hall, has been considei'ed a synonym by Dr. 

 Meek, who says that ''the type species is now known to have a 

 crenate hinge like Nucula," although the original description 

 makes it toothless. 



Nyassa, Hall, 1869. 



Syn. — Modioconcha, Hall, 1869. 



Distr. — 4 sp. Paleeozoic. N. argiUa, Hall. Devonian ; Ohio, 

 New York, 



Yalves ver^^ oblique and transversely ovate in form ; posterior 

 hinge-plate narrow, bearing from one to four long slender ridge- 

 like teeth ; anterior plate broad, marked by numerous small, 

 point-like teeth with intermediate depressions, arranged some- 

 what radiating from the middle of its inner border ; adductor 

 muscles two, one at each extremity ; pallial line entire ; ligament 

 internal. 



Ptychostolis, Tullberg, 1881. 



Distr. — Jurassic ; Nova Zembla. 



Shell obliquely oval ; hinge-plate short with about eight den- 

 ticles ; anterior to the beaks a deep lunule, behind them a small 

 escutcheon ; under the last the margin of the shell forms two 

 folds, forming a chamber communicating with the interior of the 

 shell. 



Pholadella, Hall, 1869. 



Distr. — 5 sp. Fossil ; Hamilton and Chemung Groups, N. Y. ; 

 Waverly Sandstones, Ohio. F. radiata., Conr. 



Shell equivalve, inequilateral, transversely elongated, with 

 more or less inflated valves, and strong, somewhat tumid incurved 

 beaks situated near the anterior end. Yalves crossed by a more 

 or less distinct antero-mesial sinus constricting the basal margins, 

 and also by a somewhat prominent umbonal ridge. Cardinal 

 margin inflected, forming a more or less distinctly defined 

 escutcheon and anterior lunette. Hinge characters and muscular 

 impressions unknown ; ligament external ? Surface ornamented 

 by oblique radiating ribs or striae, which are mostly confined 

 between the anterior prominence and the posterior umbonal 

 ridge ; while the anterior end and sometimes the posterior car- 

 dinal slope are without radii. 



CiMiTARiA, Hall, 1869. 

 Distr. — 3 sp. Fossil ; Hamilton Group, N. Y. C. corrugata, 

 Conr. (cix, 88). 



