154 Systematic Paleontology — Middle Devonian 



Description. — Shell small; seinielliptical to subhemispherical ; liinge- 

 line about equaling the width of the shell below. Ventral valve more or 

 less gibbous, with a longitudinal depression along the center in well 

 marked specimens; curving abniptly to the front and sides, with car- 

 dinal angles scarcely flattened; cardinal area narrow and distinctly 

 wider in the middle ; the delthyrium is small, partially closed by a 

 pseudo-deltidium, and filled by the cardinal process of the opposite 

 valve; there are generally two or three spines on each side of the center 

 of the cardinal line and in some specimens five are shown. Dorsal 

 valve follows the curvature of the ventral valve, with a lesser concavity; 

 cardinal area scarcely equal to the thickness of the shell; the interior 

 shows a longitudinal depression, and the course of the striae is well de- 

 fined and strongly papillose. Surface marked by rather strong angular 

 bifurcating striae, of which there are ten or twelve near the umbo and 

 twice as many or more on the margin; two of the striae, on each, side of 

 the center near the beak of the ventral valve, are generally larger and 

 more prominent than the others, the space between them is depressed and 

 occupied by two or three smaller striae, which are given off from the 

 larger ones on each side. 



Among the numerous specimens of Chonctes from the Romney rocks 

 of Maryland are a considerable number of a small form which agree quite 

 closely with the above description. The number of striae on the margin 

 runs up to 24 but frequently bifurcation almost at the margin consid- 

 erably increases the number. They are stronger than those of C. setiger 

 or C. scitiilus, and the impressions of some of the ventral valves appar- 

 ently show two stronger striae near the center with the depressed area 

 between them occupied by smaller striae; others, however, apparently do 

 not show this depression. There are also specimens which are difficult 

 to separate and it appears to the writer that Nicholson's opinion ' that this 

 species is the young of O. scitidus may be correct. An examination of the 

 specimen from the Marcellus shale represented by figure 12 on pi. xxii, 

 vol. iv. Palaeontology of New York, shows that there are about 36 striae 

 near the margin of the shell, while the central depressed area is no more 



*Rept. Palaeontologj' Province Ontario, 1874, p. 74. 



