192 Twenty-fourth Report on the State Museum. 



evidence of fine crenulations on one or both sides of the beaks. 

 Possessing external characters which separate it from Nucula, E'tjcu- 

 LiTEs, and Paljeoneila, it can only be referred to Tellinomya from 

 this negative information. When fully known, it will probably 

 prove to be a distinct genus, for which the name Dystaotella may 

 be used. 



Formation and locality. In the upper Helderberg formation near 

 Louisville, Ky. From the cabinet of Dr. James Knapp. 



PTYCHODESMA no^. gen. 

 Tiroxoq plicce and diafia vinculum. 



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

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

 allel lines, caused by the successive growth of the ligament, as in 

 Pectunculus. The grooves and ridges are slightly arched beneath 

 the apex of the valves where they take their origin. 



The internal hinge structure is unknown. 



Of two specimens examined, one has a length of nearly an inch 

 and a half, and a height of somewhat more than an inch ; the other 

 has the same proportions and is a little less in size. 



In general form and surface characters this genus resembles Modio- 

 MOKPHA, 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. 



The type of the genus is Ptychodesma Knajpjpiana. 



Ptychodesma Knappiana n. sp. 



Shell obliquely ovate, compressed posteriorly, and more or less 

 ventricose in the middle and toward the front ; hinge line short, 

 beaks subterminal; anterior end truncated at right angles to the 

 hinge line. 



Surface marked by fine concentric strise, with more distinct 1am- 

 inse of growth. 



Ligamental area well developed, sublinear, deeply grooved on the 

 sides, the grooves and intermediate ridges slightly inclined toward 

 the hinge line on both sides of the apex. The area shows seven 

 grooves and eight ridges on each valve; but these increase in num- 

 ber with the growth of the shell, and are, therefore, not of specific 

 value. 



This shell bears much resemblance externally to some forms of 

 MoDioMORPHA and Nyassa ; but the deeply grooved ligamental area 

 is a distinctive feature. 



