LAMELLIBRANCHIATA OF THE LOWER MARLS. 37 



tortuous and irregular in their direction, and also by finer interrupted raised 

 lines, which give a somewhat thread-like or fibrous texture to the shell. The 

 valve is flat or slightly concave exteriorly, and of somewhat smaller size 

 than the deep valve. Ligamental area of the lower valve large and strong, 

 broadly triangular, and moderately incurved, with a very distinct triangular 

 depressed central fosset. Inner cardinal margins of the shell strongly 

 crenulate. Muscular imprint large, and situated behind the center of the 

 valve. Ligamental area of the flat valve nearly vertical, and the thickened 

 margin of the shell abruptly beveled on the inside to fit the concavity of 

 the opposite valve. 



There are nearly as many varieties of this shell as there are localities 

 from which it is obtained. At least this is the case if all those usually con- 

 sidered as varieties of it are really forms of one species, which statement I 

 greatly doubt. There is not the least question but that in their present 

 condition as imperfect fossils it is extremely difficult to draw a line of spe- 

 cific demarkation between the several varieties which have been noticed 

 and referred to it, but if we had the animal to study as we have in recent 

 forms, there would be but little difficulty in recognizing several distinct and 

 well marked species. The form usually recognized as belonging to the 

 genus Pycnodonta Fischer is often so strongly marked as to give one the 

 impression when looking on its upper valve that it is more nearly related 

 to Placuna except for the cartilage pit and area. Among the various forms, 

 the following local or rather strati graphical varieties may be mentioned as 

 worthy of special notice: 



Var. convexa (Say). This form is very strongly convex on the lower 

 side, and often quite concave on the upper, with a strong anal constriction 

 separating a part of the shell near the hinge line, forming a more or less 

 perfect siphonal tube on the left side, and which usually forms considerable 

 of an angle with the hinge line. The shell is usually wider than long and 

 distinctly subangular on the umbonal ridge of the deep valve, which also 

 shows considerable obliquity. 



Var. aucella Roemer. Kreid. von Texas, p. 74, PI. IX, Fig. 4, a h. This 

 form, which is figured on PL XXVI, Figs. 9 and 10, is usually smaller and 

 quite as gibbous as G, convexa^ but has only a slight posterior sinus or con- 

 striction,^ and a deeply concave upper valve. It is also less oblique, and more 



