340 Lower Silurian Fauna of Minnesota. 



curved in and backward. Hinge long, extending from under the beaks 

 downwards toward the anterior and the posterior extremities. Ligament 

 interior, in a shallow pit under the beaks. Anterior portion of the hinge 

 marked on the inner surface by twenty teeth alternating with pits for the 

 reception of the teeth of the other valve; posterior portion likewise orna- 

 mented with twelve teeth and sockets, muscle scars two, large, deep and 

 situated at the anterior and posterior extremities of the hinge, near the 

 longitudinal extremities of the valve. Palial line simple. 



This species is closely related to T. astartaeformis Salter, but differs in 

 having a well marked lunette on the posterior cardinal slope and a longer 

 narrower area of the same kind on the anterior cardinal slope. Since 

 muscle scars are neither described nor figured with T. astartaeformis Sal- 

 ter, it is probable that the presence of deep scars is another distinguishing 

 mark for T. lepida. 



From the middle and upper part of lime stone of the Cincinnati group, 

 near Spring Valley and Granger, Fillmore county. 



About sixty very perfect free valves have bjeen found and are among 

 the exchange material in the department of Geology of the University of 

 Minnesota. 



