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1243 



TERTIARY FAUNA OF FLORIDA ^ 



inequilateral, more oval and elongate, and with more pronounced sculpture than 

 T. carolinensis. 



This group is clearl}' one which prefers relatively warm water. It appears 

 in the transitional Oak Grove sands as the cold water Chesapeake was approach- 

 ing, but disappears entirely when the latter had fully arrived. Again as the 

 Chesapeake began to give way to the milder influences which brought on the 

 Pliocene, the genus appears in the uppermost Miocene of North Carolina and 

 flourishes through the Pliocene, but retreats to Florida during the Pleistocene, 

 to advance again to the northward, after the passing of the ice age, with the 

 present fauna of the Carolinas. 



Genus TTVELA Link. 

 Tivela Link, Beschr. Nat. Samml. Rostock, ii., p. 152, 1807 {Venus corhicula Gmelin) ; 



Morch, Cat. Yoldi, ii., p. 28, 1853; Roemer, Krit. Unters. Venus, p. 15, 1857; Mon. 



Venus, i., p. i, 1864; StoHczka, Cret. Pel. India, p. 150, 1871 ; Tryon, Str. Syst. Conch., 



iii., p. 177, 1884. 

 Trigona Megerle v. Miihlfeld, Entw. neu. Syst. Schalth., p. 55, 1811 {Venus mactroides 



Born). Not Trigona Jurine, 1807, Hymenoptera. 

 Trigona Schumacher, Essai d'un Nouv. Syst., p. 153, 1817 ; Gray, Analyst, viii.. No. 24, 



p. 304, 1838; Deshayes, Cat. Conch., i., p. 45, 1853; Tate, Woodward's Man., p. 



474, 1871. 

 Trigonella Conrad, Journ. Acad. Nat. Sci. Phila., vii., p. 253, 1837 {Cytherea crassatel- 



loides Conrad). Not Trigonella Da Costa, Brit. Conch., p. 196, 1778 {^Mactra L.). 

 Pachydesma Conrad, Proc. Acad. Nat. Sci. Phila., vii., p. 31, 1854; Carpenter, Suppl. Rep. 



Brit. Assoc, 1863, p. 640 (:= Trigonella Conr., not Da Costa). 

 Eutivela Dall, Nautilus, v., p. 27, Jun^, 1891 {Eutivela perplexa Stearns). 

 Donax (sp.) Mawe, Conch., pi. ix., fig. 7, 1823. 

 fDollfusia Cossmann, Cat. Illustre bas. Paris, i., p. 116, 1886 {D. crassa Cossmann) ; 



Fischer, Man. de Conchyl., p. 1080, 1887. 



This is a very natural group with the remarkable peculiarity of having the 

 armature of the hinge variable among the different species. The cardinals, 

 especially those which are posterior, are sculptured more or less in a longi- 

 tudinal sense, with sulci or rugosities, and it would seem as if, in the species 

 with an unusual number of teeth, the excess is due to the splitting into several 

 of originally single teeth. However this may be, the dental formula ranges 

 from L. oioioioioio.oio (-^^ radiata Sowerby) to l. oioioi.oio (j-^ abaconis Dall), 

 the latter being quite a small species. The giant of the genus, T. stidtorum, 

 however, has a dental formula with only four cardinals in either valve. 



The genus has a very solid porcellanous shell, more or less trigonal and 



