NUCULID. 251 
Distr—Silurian; New York, Europe. WN. ovatus, Sowb. 
(exxvii, 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 considered 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. Paleozoic. N. arguta, Hall. Devonian ; Ohio, 
New York. 
Valves very 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. 
PrycuostTouis, 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. P. radiata, Conr. 
Shell equivalve, inequilateral, transversely elongated, with 
more or less inflated valves, and strong, somewhat tumid incurved 
beaks situated near the anterior end. Valves 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 striz, 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). 
