30 PROCEEDINGS OF THE ACADEMY OF [Part I 



The second type consists of indistinct, circular tracks. The dis- 

 tance between successive imprints is 15 mm. Running parallel 

 on the same slab, 90 mm. distant, is a similar series; the two are 

 probably associated. Their rounded outlines indicate that these 

 trails were made under water by a creature either wading or swim- 

 ming (fig. 4, no. 2). 



The third type consists of three related impressions, similarly 

 placed. The distance between impressions is 30 mm. The lack 

 of distinctness prevents a more detailed description. From their 

 appearance they were evidently produced under water (fig. 4, no. 3). 



Locality: Sandstones, Big Conewago (loo. 1 & 2). 



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ti 







Fig. 4. — Tracks. 



MOLLUSKS. 



Shells and casts of different Mollusca were found. Some of them 

 evidently represent new species. Since the literature on the Tri- 

 assic fresh-water Mollusca is meager, it was considered desirable 

 that these specimens should be identified and described by an 

 authority. In consonance with this decision, they were referred 

 to Dr. H. A. Pilsbry, whose report follows. 



The Mollusca so far found belong to the fresh-water families 

 Unionidse and Mutelidae. Species of the former family have been 

 described from the Trias of the Dockum beds of the Staked Plains 



1^ Wanner, A. The discovery of fossil tracks in the Trias of York County, 

 Pa. Second Geol. Survey of Penna., Ann. Rept., pp. 31-35, 1887. 



