17 



The first of these is a form common at various levels in the 

 Hamilton group. Nowhere, however, does it occur so 

 abundantly and so well preserved as at this level, and it is 

 especiallv in the concretionary limestone bed that this fossil 

 shows its characteristic outline and convexity of valves. It 

 is a form eminently characteristic of the Hamilton stage, 

 giving way in the Genesee to a form with few, almost 

 obsolete plications, the L. quadricostatus (Vanuxem), which 

 however, apparently did not flourish in this vicinity. Schizo- 

 holus truncatus (Hall) (Fig. 85, Pt. II.) is a characteristic 

 Genesee fossil, not commonly occurring below that forma- 

 tion. In fact, this appears to be the first locality from which 

 this fossil has been recorded as occurring in the Hamilton 

 beds, and its occurrence here is in direct accord with the slow 

 change from Hamilton to Genesee conditions which took 

 place in this portion of the Interior Devonian Sea.* It is a 

 noteworthy fact that this species has not been found in the 

 Genesee shales of this region, though it seems to be a 

 characteristic fossil of that formation in the Genesee Valley 

 and eastward. It usually occurs in the limestone bed as 

 separate valves, not infrequently showing the interior of the 

 valves. Where the true surfaces of the valves are exposed, 

 either internal or external, these commonly have a bluish- 

 gray color, which seems to be characteristic, and due to the 

 corneous character of the shell. 



Ambocoelia praeumbona (Hall) (Fig. 127, Pt. II.) while a 

 characteristic Hamilton fossil, is, in this region entirely 

 restricted to the upper part of the Moscow shales. It is an 

 abundant and well-preserved form in the concretionary lime- 

 stone bed, retaining its normal convexity in both valves. 

 The specimens vary considerably in size, and occur usually 

 as separate valves, their surface characters commonly 

 obliterated through the exfoliation of the outer lavers of the 

 shell. Brachial valves are quite as common as pedicle 



*See Chapter III, 



