ARCIDiE. 257 



Shell subquadrate, A^eutricose ; valves close, striated ; hinge- 

 teeth few and oblique, parallel with the hinge-line at each end ; 

 posterior muscular impression bounded b}' an elevated ridge. 



IDONEARCA, Courad, 1862, Shell thicker than the type, with a 

 wider hinge-plate and fewer of the short mesial denticles, which 

 are prominent and transversely striated ; lateral denticles long, 

 horizontal, or descending outAvard, and bent downward at the 

 inner ends, strongly cross-striated ; laminae of posterior muscular 

 scars prominent. Cretaceous ; Amer. and Eur. CucuUsea Tip- 

 pana, Conr. 



LATiARCA, Conrad, 1862. Shell verj^ thick and strong; hinge 

 comparatively narrow, and occupied by very irregular, strong, 

 vertical denticles, excepting at each end, where it is broad, and 

 occupied b^^ strong, rugosely-striated, longer denticles, declining 

 outward, and bent downward at the inner ends ; posterior mus- 

 cular scars, with their lower margins somewhat raised and acute, 

 but not forming a projecting lamina. CucuUaea onoclea, H. D. 

 and W. B. Rogers. Eocene. 



TRiGONARCA, Cour., 1862. Shell subtrigonal or subtrapezoidal, 

 with posterior margin obliquely truncated, and posterior basal 

 extremity more or less angular ; posterior umbonal slopes pronii- 

 nentl}^ rounded or angular ; hinge-area divaricately furrowed ; 

 hinge-teeth rather strong. C triquetra, Conr. (cxxix, 6). 



BREVIARCA, Cour., 1812. Shell small, suborbicular or suboval, 

 more or less rounded at the extremities, or rarely truncated 

 obliquely behind ; surface nearly smooth, or finely radiately 

 striated ; hinge-area with minute cross-strise ; hinge-denticles 

 very fine and crowded. G. perovalis, Courad (cxxvii, 82). 

 Cretaceous. 



Cypricardites, Conrad, 1841 ; Hall, 1868. 



Syn. — Cyrtodonta, Yanuxemia, Billings, 1858. Palfearca, Hall, 

 1859. 



Distr. — 60 sp. Silurian, Devonian ; IST. America, Wales. 



Shell ventricose, suborbicular or broad-ovate in outline, with 

 an external flattened ligamental area ; cardinal teeth four to five, 

 short, oblique ; lateral teeth two or more, oblique ; muscular 

 impressions two (anterior one single?) ; pallial line simple. 



MEGALOMUS, Hall, 1852. Shell ventricose, ponderous; beaks 

 strong, incurved, anterior; hinge-plate with several oblique 

 tuberculose teeth at the anterior end, subject to absorption ; 

 lateral teeth none ; ligament external ; anterior muscular scar 

 double, one large and deep, the other minute ; posterior scar very 

 large, obscure ; pallial line simple. Palaeozoic ; U. S. C. Cana- 

 densis, Hall (cxxix, 1). See p. 207. The genus is better placed 

 here, and the above description is the most correct. 



^GiLOPS, Hall, 1850. Described from a cast, impossible to 

 determine. Has the form of Cypricardites. 



