MYTILID^. 269 



This shell, which is not nacreous inside, is distinguished from 

 any of the Mytilidse by the form of its ligamental plates and 

 muscular impressions ; the hinge-tooth is usuall3'- overgrown and 

 nearly obliterated by the hinge-margin, as in aged examples of 

 Gardita orbicularis and Gypricardia vellicata. 



Ch^nocaedia, Meek and Worthen, 1869. 



Syn. — ? Lunulacardium (pt.), Mtinster. 



Distr. — G. ovata^ M. and W. (cxxx, 34). Fossil ; Coal- 

 measures, Ills. 



Shell equi valve ? rather thin, very inequilateral, more or less 

 oval ; beaks small, depressed and nearly terminal ; valves 

 strongly gaping in front and closed behind ; hinge unknown, but 

 short and without cardinal area ; surface with concentric stride, 

 crossed on the posterior dorsal region b}^ faintly marked radi- 

 ating costse, and on the gaping front by radiating lines and 

 costae. 



MoDiOMORPHA, Hall, 1869. 



Distr. — 9 sp. Silurian ; New York. M. concentrica, Conr. 

 (cxxix, 15\ 



Shell equivalve, ver}'^ inequilateral, compressed, subovate ; 

 beaks small, compi'essed, within the anterior third of the length ; 

 surface with rugose or undulating concentric stria?, usually coal- 

 escing or fasciculating anteriorh^ ; valves crossed obliquely by a 

 more or less distinct sinus, constricting the basal margin; 

 anterior end rounded ; hinge with a single strong wedge-shaped 

 tooth in the left valve and a corresponding cavity in the right 

 valve.; no lateral teeth ; ligament external, attached to the thick- 

 ened, margin of the shell ; pallial line entire. 



MomoLiNA, Meek, 187Y. ilf. Za^a, Meek. Upper Trias ; Buena 

 Vista Canon, W. Humboldt Range, United States. Not char- 

 acterized. 



Ptychodesma, Hall and Whitfield, 1872. 



Type, P. Knappiana, H. and W. Palaeozoic; "Hydraulic 

 Beds," Louisville, Ky. 



Shell modioloid in form ; valves equally convex ; hinge with a 

 wide ligamental area, the sides of which are sharply grooved in 

 parallel lines, caused by the successive growth of the ligament 

 as in Planorbis ; the grooves and ridges are slightly arched 

 beneath the apex of the valves, where they take their origin ; 

 internal hinge-structure unknown. 



Resembles Modiomorpha in form and surface characters, but 

 differs in having a ligamental area. Externally it is unlike 

 Cypricardites and similar shells, which have a ligamental area 

 marked by fine striae parallel to the hinge-line, while these are 

 parallel to the margin of the shell. 



