202 
Kirtley F, Mather 
Composita ovata n. sp. 
Plate XIV, figures 6-6c. 
Description. Shell medium size, sub-oval in outline, width 
typically much less than length but approaching the latter 
dimension in some individuals, greatest width more or less an- 
terior to middle, convexity of valves sub-equal and comparatively 
low. The dimensions of two nearly perfect individuals are: 
length, 27.3 mm., 21.8 mm. ; width, 26.8 mm., 18.0 mm. ; thick- 
ness, 13.7 mm., 10.7 mm. 
Pedicle valve broadly convex, curving sharply from the umbo 
toward the cardinal margin but sloping gently toward the lateral 
and anterior margins; mesial sinus originating in the umbonal 
region about one-fourth the distance from beak to anterior 
margin as a narrow, well-defined sulcus which continues to the 
margin with little or no increase in depth or width; the mesial 
portion with a breadth about two-fifths that of the greatest 
width of the valve, produced anteriorly and with little or no 
transverse curvature, the longitudinal convexity continuing 
without notable change to the anterior margin of the extension 
which has a broadly spatulate outline ; beak prominent, incurved 
above that of brachial valve, truncated by a large, circular fora- 
men encroaching upon the umbo; delthyrium concealed. 
Brachial valve more strongly curved transversely than longi- 
tudinally, greatest convexity near the umbo; mesial fold broad 
and undefined, originating in the umbonal region and increasing 
in width anteriorly until it occupies about two-fifths the width 
of the valve, gently rounded on top with lateral slopes increas- 
ing slightly in steepness and height toward the anterior margin ; 
beak somewhat tumid, incurved over the delthyrium of the 
pedicle valve. 
Surface of both valves covered by fine, close-set concentric 
lines of growth with occasional lines much stronger than the 
intermediate ones and crossed by fine thread-like radiating 
striae. 
Remarks. Individuals included in this species vary consider- 
ably in outline from longitudinally oval to nearly sub-circular but 
all are characterized by the comparatively low longitudinal con- 
vexity resulting from the slight thickness of the shell. The 
absence in the pedicle valve of the broad mesial sinus bordered 
