SYNOPSIS OF GENERA. xlv 



Orthonota, Conrad (Geol. Surv. N. Y., Annual Report on the Pateontological 



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 O. 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 



