226 Monograph of the Lamellihranchiata. 



Genus Lyrodesma — (Conrad, 1841). 



Equivalve, inequilateral ; trigonia shaped, elongated, posterior 

 area striated, hinge with from five to nine diverging prominent cardi- 

 nal teeth, transversely striated ; ligament external. 



Lyrodesma plana — (Conead, 1841). 



Subrhomboidal, compressed; posterior margin widely and obtusely 

 truncated ; posterior basal margin rectilinear and striated ; extremity 

 rounded ; cardinal process semi-circular, and marked by eight diverg- 

 ing teeth. 



This fossil is found at the quarries back of Cincinnati. It is quite 

 rare, and its range is unknown. 



* 



Lyrodesma Oincinnatiensis — (Hall, 1871). 



Shell small, subrhomboidal in outline, and obtusely pointed at the 

 posterio- basal angle; valves moderately convex with a subangu- 

 lar umbonal ridge and narrow cardiiial slope; anterior end rounded 

 and passing into the more broadly rounded basal line; posterior end 

 oblique, pointed below ; hinge line short ; beak very small. Hinge 

 plate occupied by six angular, crenulated, radiating teeth, which, diverg- 

 ing from beneath the beak, are strongly arched upward between their 

 origin and extremities; crenulations minute, but very distinct ; muscu- 

 lar impressions and pallial line not observed. 



Differs from L. postriata and L. plana in the much shorter form 

 and in number of teeth. 



Found at different places, within 300 feet of low water-mark. It is 

 a rare fossil, and the extent of its range is not known. 



Genus Anodontopsis — (McCoy). 



Equivalve, inequilateral, compressed ; general form rotundato-quad- 

 rate or subtrigonal ; posterior side wide; round, or obliquely sub- 

 truncate, anterior end slightly contracted in front of the beak ; beaks 

 small, prominent, nearer to the anterior than the posterior end ; hinge 

 line shorter than the shell, with a posterior, long, slender, lateral tooth 

 or cartilage plate, extending just below it (double in the right valves), 

 and another similar but shorter one in front of the beaks ; anterior and 

 posterior muscular impressions, simple, ovate, the latter longer and 

 stronger than the anterior ; occasionally a slight clavicular I'idge ex- 



