CARDINIIDA. 235 
Shell rather compressed, subtrigonal or trapezoid, anteriorly 
generally somewhat narrowly rounded, posteriorly obliquely trun- 
eate, hinge rectilinear, shorter than the length of the shell, with 
an internal posterior marginal tooth, double in the right valve, 
and a second shorter one in front of the beaks; a small cardinal 
tooth is said to be occasionally present below the beaks; mus- 
cular scars ovate, the posterior larger and stronger than the 
anterior; pallial line entire. 
Stoliczka considers Microdon, Conrad (p. 191), identical with 
Anodontopsis. 
PSEUDAXINUS, Salter. Shell thin, edentulous, convex, with 
prominent umbones, and a strong posterior carinated edge ; beaks 
anterior; no lunette. A. securiformis, M’Coy, and allied forms. 
ORTHODONTISCUS, Meek, 1873. Right valve with a well-defined 
cardinal tooth, left valve with a corresponding fosset and a rudi- 
mentary tooth in front of it; lateral teeth double in the right 
valve, single in the left. A. Milleri, Meek. Silur.; Ohio. 
PacuycarpiA, Hauer, 1857. 
Distr.—P. rugosa, Hauer. Alpine Trias. 
Shell long-oval, nearly trigonal, very inequilateral, concen- 
trically striated or smooth; beaks prominent, approaching, 
nearly terminal; anterior side strongly swollen, with lunule; 
posterior side somewhat compressed ; margins smooth, the ven- 
tral convex in outline; ligament external, short; teeth two in 
each valve, strong, diverging, the anterior in the right valve 
less developed and nearly marginal; a long, posterior, lateral 
tooth in each valve; muscular impressions small, the anterior 
deep. 
Orper ASIPHONIDA. 
Mantle-margins open; no siphons; pallial impression without 
sinus. 
Suborder HomomyariA. Mantle-margins open or closed behind ; 
both muscular impressions equally distinct. 
Suborder HereroMyaria. Frequently inequivalve; anterior 
muscular impression very small, posterior impression large. 
Suborder Monomyaria. Having a single posterior or sub- 
central adductor-muscle and impression. 
SuporpeR HOMOMYARTIA. 
( Cardiniacea.) 
Famity CARDINIID A. 
Shell obliquely lengthened or oval, smooth or concentrically 
striated; ligament external, moderately long; cardinal teeth 
