ARCIDA. 257 
Shell subquadrate, ventricose; valves close, striated ; hinge- 
teeth few and oblique, parallel with the hinge-line at each end ; 
posterior muscular impression bounded by an elevated ridge. 
IDONEARCA, Conrad, 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 outward, and bent downward at the 
inner ends, strongly cross-striated ; lamina of posterior muscular 
scars prominent. Cretaceous; Amer. and Eur. Cuculleza Tip- 
pana, Conr. 
LATIARCA, Conrad, 1862. Shell very thick and strong; hinge 
comparatively narrow, and occupied by very irregular, strong, 
vertical denticles, excepting at each end, where it is broad, and 
occupied by 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 2 projecting lamina. Cucullza onoclea, H. D. 
and W. B. Rogers. Hocene. 
TRIGONARCA, Conr., 1862. Shell subtrigonal or subtrapezoidal, 
with posterior margin obliquely truncated, and posterior basal 
extremity more or less angular; posterior umbonal slopes promi- 
nently rounded or angular; hinge-area divaricately furrowed ; 
hinge-teeth rather strong. C. triquetra, Conr. (cxxix, 6). 
BREVIARCA, Conr., 1872. 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-strie; hinge-denticles 
very fine and crowded. C. perovalis, Conrad (cxxvii, 82). 
Cretaceous. 
CypricaARDITES, Conrad, 1841; Hall, 1868. 
Syn.—Cyrtodonta, Vanuxemia, Billings, 1858. Palearca, Hall, 
1859. 
Distr.—60 sp. Silurian, Devonian; N. 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. Paleozoic; U.S. C. Cana- 
densis, Hall (exxix, 7). 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, 
