INSTITUTE OF SCIENCE, PHILADELPHIA. 101 



interior faces of the valves ; hinge-area, in the adult, greatly elongated, 

 either straight or laterally twisted, sometimes nearly one-half the length 

 of shell ; the median groove of variable width and depth, projecting 

 below the lateral ridges, so as to make the hollow of the shell appear 

 two-horned ; cavity of shell shallow, impressed medially just below the 

 hinge-line ; muscular scar semilunar, deep. 



Length (height), 6.3 inches ; width, variable ; thickness of shell, two 

 to two and a half inches and upward. 



Very abundant in the " marl " banks below Thorpe's, where the shell 

 can be seen several feet below the surface of the water ; less abundant in 

 the banks below Fort Thompson, and at one or two points between this 

 locality and Thorpe's. 



This form may be readily distinguished from all the other Eastern 

 species of Ostrea by its ponderous proportions, greatly surpassing in this 

 respect 0. percrassa from the Miocene of New Jersey. In its early 

 stage it resembles in both outline and ornamentation the recent 0. borcalis, 

 but the adult form, from its manner of growth, is wholly different. 



The following additional species of Mollusca were obtained from the 

 deposits below Fort Thompson : 



Nassa vibex, Say. 



Journ. Acad. Nat. Sciences Phila., ii, p. 231. 



Crepidula cymbaeformis, Conr. 



Proc. Acad. Nat. Sciences Phila., ii, p. 173. 



Crepidula fornicata, L. 



Syst. Nat., p. 1257. 



Bulla striata, Brug. 

 Diet., No. 3. 



Siliqua bidentata, Spengler. 



Skrivt. Nat. Selsk., iii, p. 104. 



Semele variegatum, Lam. 



Anim. s. Vertebr., vi, p. 126. 



Eangia cyrenoides, Des M. 



Actes Soc. Linn. Bordeaux, v, p. 57. 



Venus cancellata, L. 



Syst. Nat., p. 1130. 



Venus Eileyi, Conr. 



Foss. Med. Tert. U. S., p. 9, vi, fig. I. 

 Venus Mortoni, Conr. 



Journ. Acad. Nat. Sciences Phila., vii, p. 251. 



Artemis discus, Reeve. 



Conch. Icon., Artemis, 9. 



Artemis elegans, Conr. 



Foss. Med. Tert. U. S., p. 30. 



Dione (Calliste) gigantea, Chem. 

 Conch. Cab., x, p. 354. 



