TRENTON CONGLOMERATE OF RYSEDORPH HILL 81 



gin stronglj' rounded, veatral margin more gently curved, jios- 

 terior margin lei&s curved, truncated above; sulcus in about the 

 middle of the valve, deep ventrally, nol; reaching to the middle 

 of the valve, curving slightly backward in the lower part; dorsal 

 parts on both sides of the sulcus somewhat bulbous, the anterior 

 part more distinctly so. Well preserved specimens show a flat 

 border sloping steeply outward. Internal casts (fig. 5) possess 

 a prominent tubercle at the inner end of the sulcus, evi- 

 dently indicative of a deep muscle impression. Surface very finely 

 granulose, the granules arranged in longitudinal rows, so that 

 with a weaker glass the surface appears to be marked by longi- 

 tudinal, anastomosing striae. 



Dimensions. Length .7 mm; hight .5 mm; thickness .2 mm. 



Horizon and locality. In the pebbles of gray crystalline lime- 

 stone and of black compact limestone, Rysedorph hill. (Groups 

 5,7) 



Observations. These minute fossils show considerable similar- 

 ity to P. m u n d u 1 a S. A. Miller, from the upper half of the 

 Cincinnati group. They differ in being a little larger, 

 in having the tumid dorsal parts more strongly developed, and 

 specially in having a finely granulose surface. In consideration 

 of the variability of P . m u n d u 1 a , shown by Jones 

 and Ulrich, it is very probable that this form is only an earlier 

 variety of the Cincinnati type. Jones's P. 1 o gan i, from the 

 Canadian Chazy (?) and Trenton, has the front moiety mostly 

 narrower than the hinder part and is smooth and punctate, 

 while his P. mundula var. e f f o s s a from the dark and 

 fine grained limestone from the south of Montcalm market, Que- 

 bec city, is a form quite similar to ours, but differing still in 

 being longer, having the sulcus developed in the center into a 

 large pit and in being smooth. It occurs in beds which Lap- 

 worth and Ami have shown toi contain the fauna of the Normans 

 kill beds, in which the Rysedorph hill conglomerate is embedded, 

 so that both these varieties of P. mundula are of older 

 Trenton age. 



