SYNOPSIS OF GENERA. 
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Orthonota, Conrad (Geol. Surv. N. Y., Annual Report on the Palaeontological 
Department, p. 50. 1841). 
[Type, Orthonota undulata.] 
“ Equivalved, profoundly elongated. Hinge and basal margins straight and 
parallel. Beaks near the anterior extremity; posterior extremity truncated.” 
The above is Mr. Conrad’s description. The shells, in all the species referred 
to the genus, are extremely elongate and inequilateral; the anterior end is 
rounded and the posterior end truncate. The beaks are near the anterior end. 
The cardinal line is straight or sometimes slightly concave, extending nearly 
the entire length of the shell. The umbonal slope is rounded or angular, and 
defined by one or more distinct folds, which extend to the post-basal extremity. 
Surface marked by fine striae of growth, and in the typical species by strong 
undulations on the post-cardinal slope and less conspicuously on the anterior 
end. Two or more defined oblique folds, which may be rounded or angular, 
extend from the umbones to the posterior and post-basal margins. 
Hinge characters unknown; no teeth or crenulations have been observed. 
Cardinal margin with a long linear fold embracing the ligament, which is 
apparently internal. The anterior end is without lunule, and the margins of 
the valves continue in a straight line beyond the beaks. The muscular impres¬ 
sions are obscure, and have not been satisfactorily determined. Pallial line 
undetermined. 
The few species of this genus yet absolutely determined form a very distinct 
group, and, so far as known, are restricted to the Hamilton and Chemung 
groups. The first species designated by Mr. Conrad (Orthonota pholadis), from 
the Hudson River group, is known to us only by a figure of the species, and 
this is evidently not congeneric with 0. undulata, which Mr. Conrad always 
regarded as the typical species of the genus. 
The characteristics of the genus have not always been fully understood or 
appreciated by palaeontologists, and the inclusion of forms allied to, or identical 
with, Grammysia by several European authors has served to mislead. The 
species of this genus are always elongate, extremely inequilateral, without 
