CARDINIID^. 235 



Shell rather compressed, subtrigonal or trapezoid, anteriorly 

 generally somewhat narrowly rounded, posteriorl3^ obliquelj' trun- 

 cate, 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 occasionall}^ 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. secuHformis, 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. 31illeri, Meek. Silur. ; Ohio. 



Pachycardia, 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. 



Order ASIPHONIDA. 



Mantle-margins open ; no siphons ; pallial impression without 

 sinus. 



Suborder Homo my aria. Mantle-margins open or closed behind ; 

 both muscular impressions equally distinct. 



Suborder Heteromyaria. Frequently inequi valve ; anterior 

 muscular impression very small, posterior impression large. 



Suborder Monomyaria. Having a single posterior or sub- 

 central adductor-muscle and impression. 



Suborder HOMOMYARIA. 

 {Gardiniacea.) 



Family CARDINIIDiE. 



Shell obliquely lengthened or oval, smooth or concentrically 

 striated ; ligament external, moderately long ; cardinal teeth 



