Vol. VII] DICKERSON & KEW— FAUNA MEDIAL TERTIARY 129 



Conrad, Melongcna, sp., Natica, sp. Of these, Ficiis missis- 

 sippiensis has been found from the Vicksburg horizon and 

 from the Bowden beds; Pecten gatiinensis is a characteristic 

 form from the Panama Canal Zone. Thus Agassizia clevei 

 and Lovenia dumhlci are proved to be associates of these 

 characteristic species of the Bowden stage and representative 

 fossils of the medial Tertiary of Mexico. 



We are now able to determine the species characteristic of 

 the San Fernando beds by using Agassizia clevei, Lovenia 

 dumblei, Macropneustcs antillanun, Clypeaster cubeiisis, Pec- 

 ten gatiinensis, and Pecten condylomatus as our guide fossils. 

 The forms specifically determined are as follows : Agassizia 

 clevei, Clypeaster ciibensis, Lovenia dumblei, Macropneustes 

 antillarum, Metalia cumminsi, Schizaster scherzeri, Area trini- 

 taria, Antigona glyptoconcha, Cardium gatunense, Cardiiun 

 lingua-leonis, Ostrea Jiaiticnsis, Ostrea trigonalis, Pecten gatun- 

 ensis, Pecten condylomatus, Pecten oxygonum optimum, Pec- 

 ten levicostatus, Conns interstinctus, Ficns mississippiensis, 

 Malea ringens and Turritella altilira. The great number of 

 echinoids and their widespread distribution is due in part at 

 least, to deposition near the shore-line. All the above echinoids 

 should prove to be good horizon markers when carefully deter- 

 mined. Macropneustes antillarum and Agassizia clevei were de- 

 scribed from beds of supposed Eocene age of St. Bartholomew 

 and Cuba, but their association here with forms characteristic 

 of the Bowden horizon proves that they are members of this 

 fauna. 



Conclusions 



( 1 ) The fauna of the medial Tertiary beds of northeastern 

 Mexico is a littoral facies of the Bowden or Chattahoochee 

 horizons. 



(2) Clypeaster cubensis, Agassizia clevei, Lovenia dumblei, 

 Pecten condylomatus, Pecten oxygonum optimum, Pecten 

 gatiinensis, are the best guide fossils of the medial Tertiarv. 



