ARCID^. 255 



ribs, and narrow intervening furrows ; epidermis smooth, olive ; 

 teeth large, oblong, arcuated on each side. A. senilis, Linn. 

 (cxxvi, 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 (cxxvi, 46, 47). 



ISOARCA, 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. I. texta, 

 Miinst. (cxxvii, 69). 



CARBONARCA, Meek and Worthen, 18Y5. Shell (as determined 

 from internal casts) eqilivalve, inequilateral, very convex, trans- 

 versely oblong or oval ; umbones gibbous, prominent, and sti'on^ly 

 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 Area. 

 C. gibbosa, M. and W. f cxxvii, 11). 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 (cxxvi, 49). 



NOETiA, Gray, 1842. Shell trigonal, ventricose, inequilateral, 

 equivah^e ; posterior side truncate, strongly angulate ; teeth 

 smallest in the middle, anteriorly elongated, posteriorly arcuated. 

 *A. ponderosa, Say (cxxviii, 84,85). 



GLYPTARCA, 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 mu.scular impression strong, posterior less 

 distinct ; hinge-area narrow, plate thick, with three teeth in front 

 of the umbo ; surface strongly marked with growth-lines. 2 sp. 

 Silur. ; Wales. A. primaeva, Hicks (cxxvi, 37, 88). 



Macrodon, Lycett, 1845. 

 Syn. — Parallelodon, Meek and Worthen. 



Distr. — Oolite — Recent. M. asperula, Dall. Yucatan. Fossil. 

 M. Hirsonensis, d'Arch. (cxxvii, 72). 



