530 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Discussion.—Members of this genus are fairly common at a few places where 
early Middle Ordovician rocks are exposed. Generally these specimens have 
been placed in the genus Camerella. Examination of the pedicle beak region and 
confirmation of the presence of divergent dental plates definitely rules out rela- 
tionship to the camerellids. The entire structure of this genus places it as the 
earliest known member of the Triplesiidae. 
Triplesid characters appear in the pedicle valve in the form of the character- 
istic pseudodeltidium. This is a nearly flat plate narrowly folded in the middle. 
This narrow fold appears to be produced by anterior forward growth of the 
pseudodeltidium over the small triangular chilidium that overlies the shaft of 
the cardinal process. A few specimens only have revealed the presence of a 
small foramen at the apex. This may be elongated along the umbo by resorption 
as has been shown for other triplesids. 
The brachial interior of most of the species described herein has been proved 
to possess the forked cardinal process of the Triplesiidae. In addition to the 
cardinal process several of the species revealed a small triangular chilidium like 
that described for Onychotreta. 
The appearance and characters of this genus place it closest to Triplesia, and 
in common with that genus it is smooth and uniplicate. It differs from Triplesia 
generally in its smaller size, usually disproportionate depth of valves, the brachial 
valve being several times deeper than the pedicle valve, the extremely narrow 
hinge, and the elongated beak. Early members of the genus suggest juveniles of 
Triplesia. 
Onychoplecia is first seen in the Table Head series of Newfoundland in a small, 
strongly folded species. It is fairly common in Chazy rocks (Crown Point) of 
New York. It is less common in Marmor rocks of Pennsylvania and Tennessee. 
The youngest specimens known are from the Lower Bromide of Oklahoma and 
the Sevier of East Tennessee, the latter from about the Benbolt level probably 
being the younger. 
ONYCHOPLECIA BREVIROSTRIS Cooper, new species 
Plate 99, K, figures 65-70; plate 100, D, figures 15-22; plate 100, F, figures 28, 29 
Shell small, but of fairly large size for the genus, tear-shaped in outline, com- 
pressed lenticular in profile; sides rounded and with greatest shell-width located 
anterior to the middle; front margin somewhat truncated. Anterior commissure 
more or less broadly uniplicate. Surface marked by fine concentric lines of 
growth. 
Pedicle valve subtriangular in outline with the anterolateral margins strongly 
rounded but the posterolateral margins nearly straight and making an angle of 
85°. Lateral profile gently convex in the posterior two-thirds, moderately con- 
vex in the anterior third. Anterior profile broadly and moderately convex. Sul- 
cus broad and shallow, confined to the anterior third; tongue short, broadly 
rounded. Median region and flanks swollen, interarea moderately long, pseudo- 
deltidium with prominent and fairly wide elevated ridge. 
