222 Monograph of the LameUibranchiafa. 



Orthonota contracta — (Hall, 1847). 



Subcylindrical, slightly arcuated; beaks distinct, acute, with a 

 prominent oblique carina extending toward the posterior basal n rgin, 

 which is arcuated, and the shell much contracted below and p( lerior 

 to the beaks ; dorsal margin broad, rounded, the valves strongly 

 marked by oblique folds. 



The base is arcuate, and distinctly contracted or sinuate a little be- 

 hind the beaks. It differs from 0. parallela, in having more acute 

 and distinct beaks, with cardinal margin broader. 



This fossil is known only as a cast. It is scarce at the quarries, and 

 its range is unknown. 



Orthonota pholadis — (Conead, 1838). 



Shell profoundly elongated, ventricose ; dorsal and basal margins 

 parallel ; posterior side rugose, or with short undulations near the 

 dorsal margin. Length, If inches. 



The form believed to belong to this species is quite rare at the quar- 

 ries back of Cincinnati, and is found only as a subcylindrical cast. It 

 range is not known. 



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Genus Cleidophorus — (Hall, 1847). 



\_Cleidophorus, in allusion to the clavicle in each valve, anterior to 

 the beak]. 



Shell equivalve, inequilateral ; hinge without teeth or crenulations ; 

 surface (particularly in casts), marked by an oblique linear depres- 

 sion, extending from the anterior cardinal margin toward the base, 

 indicating the place of the clavicle ; surface concentrically striated. 



Cleidophorus fahida — (Hall, 1845). > 



Shell minute, or very small, transversely subelliptic, mo derately 

 convex; extremities narrowly rounded, the anterior end being nar- 

 rower than the posterior ; basal mai'gin forming a broad semi-elliptic 

 curve ; beaks rather depressed, slightly tumid, and j^laced a little in 

 advance of the middle ; dorsal margin sloping gently from the beaks, 

 the anterior slope being rather less gradual than the other, and, in the 

 cast, a little concave in front of the beaks. Anterior muscular impres- 

 sions distinctly defined by the internal ridge, which leaves a rather 

 deep furrow just in advance of each beak in casts of the interior. 



Length of a medium sized specimen, 0.06 inch ; heigiit, 0-03 inch ; 

 convexity about 0.02 inch. 



