570 PROCEEDINGS OF THE ACADEMY OF [1896. 



over, they are better developed. Still, the characteristic Mississippi 

 Kiver types of Unionidce are not represented in the Fish-house fauna. 

 None of the triangular or round Unios with heavy teeth are found ; 

 no member of the great tuberculate or plicate groups occur. The 

 Fish-house fauna is therefore to be assimilated rather with the Great 

 Lake system than with the Mississippi or Ohio drainages. The spe- 

 cies probably found their way into the Atlantic system in New 

 York State, where the Lake and Atlantic waters are in close prox- 

 imity. They may then have become extinct on the Atlantic slope 

 during the glacial period when the rivers north of Delaware Bay 

 wex'e so profoundly affected.' 



Summary. — The writer has attempted to show (1) that the Fish- 

 house clay is a Pleistocene deposit, as held by Lewis, White and 

 some others, not belonging to the Cretaceous or Tertiary as Lea, 

 Whitfield and other geologists have claimed; (2) that it is either 

 interglacial or preglacial, and probably the latter ; (3) that it 

 is purely local and fluviatile ; and (4) that the structure of the 

 sand underlying the clay, now first made known, gives a clue to the 

 true explanation of the several geologic features of the deposit. 



The position of this deposit in the post-Pliocene series is one of 

 some difficulty, but materials bearing upon the question are not 

 wanting. We know that the immediately post-glacial mollusk fauna 

 of New Jersey was similar to the modern, except that it contained 

 forms of more northern distribution ; but there were no distinctively 

 trans-AUeghenian types such as the Fish-house beds contain.* The 

 very difl^erent character of the latter fauna would therefore indicate 

 an earlier period. It was therefore either interglacial or preglacial, 

 and the divergence of a part of the species from the most allied 

 living forms, as well as the fact that the fauna was an abundant one, 

 composed of large and well-developed individuals, point rather to 

 preglacial than to interglacial conditions. 



' Those interested in the former distribution eastward of the trans-AUe- 

 ghenian Unionidse should consult Simpson, On some Fossil Unios from the 

 Drift at Toronto, Canada. Proc. U. S. Nat. Mus., XVI, p. 591. 



* White Pond, in Sussex Co., N. J , a typically glacial lake, furnishes abund- 

 ant evidence in support of the above statement, and also shows the changes 

 which have taken place from post glacial to recent times in the mollusk fauna. 

 This evidence the writer proposes to publish as soon as engagements permit. 



