ARCID. 955 
ribs, and narrow intervening furrows; epidermis smooth, olive ; 
teeth large, oblong, arcuated on each side. <A. senilis, Linn. 
(exxvi, 45). 
ARGINA, Gray, 1840. Shell subglobose, subcordiform, equi- 
valve, inequilateral ; covered with radiating ribs and a brown, 
hairy epidermis; teeth numerous, elongated, curved, the central 
ones very small. A. pexata, Say (exxvi, 46, 47). 
1soarnca, Miinster,1842. Shell ventricose; beaks large,anterior, 
often subspiral; ligament entirely external; hinge-line curved, 
with transverse teeth, smallest in the centre; pallial line simple. 
I. Logani (Ctenodonta), Salter, Lower Silurian, Canada, is 3 
inches long, and has the ligament preserved. Fossil, 14 sp. 
Lower Silurian—Cretaceous ; North America, Europe. JI. fexta, 
Miinst. (exxvii, 69). 
CARBONARCA, Meek and Worthen, 1875. Shell (as determined 
from internal casts) equivalve, inequilateral, very convex, trans- 
versely oblong or oval; umbones gibbous, prominent, and strongly 
incurved with subangular or prominent posterior slopes ; valves 
closed all around, with smooth margins; ligament external ; 
cardinal margin a little arched, with, at the anterior extremity 
in each valve, two rather oblique comparatively stout teeth, and 
extending along its entire length from immediately behind these, 
a row of minute, interlocking teeth or crenulations as in Arca, 
C. gibbosa, M. and W. (cxxvii, 71). Upper Coal-measures, Ills. 
LUNARCA, Gray, 1842. Shell globose, subcordiform, nearly 
equivalve; surface radiately costate, with brown, foliaceous 
epidermis; hinge with the posterior tooth elongated ; narrow in 
the middle; the front tooth of the left valve ovate; elevated, 
entire, fitting into a cavity in the inner edge of the front margin 
of the right valve. A. costata, Gray (exxvi, 49). 
NOETIA, Gray, 1842. Shell trigonal, ventricose, inequilateral, 
equivalve; posterior side truncate, strongly angulate; teeth 
smallest in the middle, anteriorly elongated, posteriorly arcuated. 
A. ponderosa, Say (exxviii, 84,85). 
GLyprarca, Hicks, 1872. Inequilateral, strongly ventricose ; 
beak near the anterior end, prominent, overhanging the hinge- 
line more or less and pointed at the extremity; two diverging 
ridges extend from the umbo to the margin, and enclose a trian- 
gular sulcus, having its base at the margin, which it thereby 
indents; anterior muscular impression strong, posterior less 
distinet ; hinge-area narrow, plate thick, with three teeth in front 
of the umbo; surface strongly marked with growth-lines. 2 sp. 
Silur.; Wales. A. primeva, Hicks (cxxvi, 37, 38). 
Macropon, Lycett, 1845. 
Syn.—Parallelodon, Meek and Worthen. ! §66. Chic. He. Proc, 
Distr.—Oolite—Recent. M. asperula, Dall. Yucatan. Fossil. 
M. Hirsonensis, d’Arch. (cxxvii, 72). 
