GUELPH FAUNA IN THE STATE OF NEW YORK 47 



lateral margins. The plications are' not quite as prominent as in the typical 

 Rochester shale specimens. 



This shell is widely distributed in the Niagaran of North America, 

 where it occurs in the Rochester shale, Waldron and Osgood beds ; but it 

 seems to be absent from the higher beds of the Niagaran, and has not been 

 observed before in the Guelph. Its appearance, though extremely rare, in 

 the Guelph of Oak Orchard creek, is hence worthy of notice. 



LAMELLIBRANCHIATA 



MYTILARCA Hall. 187O 



Mytilarca eduliformis sp. nov. 



Plate s, fig. 8-10 



Shells rather small, valves ovate acuminate, very narrow at the beaks, 

 with slightly concave anterior margins, broadly rounded at the base and at 

 the postlateral extremity, the posterior cardinal margin being straight. 

 Beaks narrow, subacute and directed forward. Surface elevated along the 

 umbonal ridge which runs from the beaks to the antelateral curve. From 

 this ridge the surface is abruptly incurved and almost vertical. Posteriorly 

 the slope is very much more gradual, and the ridge loses its prominence 

 over the basal region. The ornament is not well preserved, but patches of 

 the shell show fine concentric lines without other modification. 



Dimensions. This description is based on two specimens, one of which 

 retains the valves in normal juxtaposition. The valves have a length of 

 20 mm and a width at three fourths their length of 1 5 mm. 



Observations. This shell has a noteworthy resemblance in form and 

 contour to a small example of the living M y t i 1 u s edul i s. It is pro- 

 visionally referred to the genus Mytilarca, though some generic distinction 

 may eventually be found between this and the typical upper Devonic repre- 

 sentative of the genus. The only American Upper Siluric species which 

 has heretofore been referred to Mytilarca is the M. s i g i 1 1 u m Hall from 

 the Niagaran at Waldron Ind. 



Mytilarca eduliformis is from the white chert at Rochester. 



