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1307 



TERTIARY FAUNA OF FLORIDA 



Mercenaria Schumacher, Essai, pp. 45, 135, 1817 {V. mercenaria L.) ; Morch, Yoldi Cat., 

 ii., p. 23, 1853 ; Romer, Krit Unters., p. 16, 1857 ; H. and A. Adams, Gen. Rec. Mol., 

 ii., p. 419, 1857; Stoliczka, Cret. Pel. India, p. 153, 1871 ; Tryon, Syst. Conch., iii., p. 

 176, 1884. 



Crassivenus Perkins, Proc. Boston Soc. Nat. Hist., xiii., p. 147, 1869 (V. mercenaria 

 Linne). 



Not Mercenaria Cossmann, Cat. Illustre, p. 94, 1887. 



The genus Venus, as restricted, is a very compact and homogeneous group 

 illustrating the highest development of the hinge-structure and the most ex- 

 treme limit of size afforded b_v the genus in its widest sense. While not afford- 

 ing such exemplars of beauty in color and sculpture as the tropical groups 

 contain, nevertheless the reputation of the species as a basis for Indian trade 

 and a very important food supply is worthy of its distinction as type of the 

 most characteristic product of evolution in the Pelecypoda. 



The shell of Venus is solid and heavy, porcellanous, and somewhat earthy ; 

 the periostracum extremely thin and hardly visible ; the form is rounded or 

 trigonal with faint radial striation and stronger concentric lamellosity ; the 

 inner margins are crenulate ; the pallial sinus is small and triangular ; the 

 beaks are prominent ; the lunule and escutcheon well defined ; there are two 

 bifid cardinal teeth in the left valve ; one posterior bifid and two anterior simple 

 cardinals in the right valve ; a supplementary posterior cardinal in each valve 

 below the ligamentary nymph is modified to form a rugose area of which the 

 asperities interlock with those of the opposite valve. The genus is represented 

 on muddy or sandy bottom in shallow water from the north shore of the Gulf 

 of Mexico to Cape Cod, with some still more northern colonies reaching the 

 Gulf of St. Lawrence ; a single species is found in the Oregonian region. The 

 group appears first in the Oligocene and seems to have had its maximum de- 

 velopment in the Miocene. As far as yet known it is confined to North America 

 and Japan. 



Venus halidona Dall. 

 Plate 38, Figures i, la. 

 Venus halidona Dall, Trans. Wagner Inst, iii., part v., p. 1194, pi. xxxviii., figs, i, la, 1900. 

 Oligocene silex beds of Hillsboro' Bay and Ballast Point near Tampa, 

 Florida ; Dall, Burns, and Willcox. 



Shell small for the genus, subovate, slightly truncate behind ; beaks low, 

 anteriorly directed over a rather large lanceolate lunule ; posterior dorsal area 

 narrow, nearly smooth, elongate, laterally keeled; sculpture of sharp, thin, 



