Age of the Trinity Division. . 17 



Viviparus natica (?) cossatotensis Hill. 

 Cylindrites (?) sp. indet. 

 Buccinopsis (?) parryi Conrad. 

 Tylostoma pcdernalls (Roemor). 

 Vicarya branneri sp. nov. 

 Nerinsea austinensis Roemer. 

 Neritina sp. indet. 

 Ncumayria walcotti Hill. 

 Acanthoceras (tyjustinse sp. nov. 



Arthropoda : 



Oypridea tc.rana sp. nov. 



Vcrtcbrata : 

 Lepidotus. 



Crocodila. 



Dinosauriia. 



Chclonia. 



Plantse : 



Many species, now in course of publication by Professors 

 Ward and Fontaine, together with undetermined species 

 resembling Araucarites, figured on plate 1. 



IV.- AGE AND SIGNIFICANCE OF THE TRINITY DIVISION. 



With the exception of the genera Requienia and Monopleura, 

 the above list contains none of the hitherto familiar types of fos- 

 sils found in the overlying Eredericksburg and Washita Divisions, 

 such as the characteristic Eckinodermata, Radiolites (alleged Hip- 

 purites), Gryphons of the Pitcher i group, Exogyras, or Schloenbach- 

 iale Ammonitidw, but possesses a molluscan fauna peculiarly its 

 own, so far as America is concerned, which, accompanied by a 

 well-preserved flora and vertebrate fauna, affords the first satis- 

 factory and complete data for an age classification of the sub- 

 divisions of the North American Lower Cretaceous formation, 

 and will enable us to more thoroughly interpret the succeeding 

 divisions. This association of vertebrates, plants, and marine 

 mollusca is a most important fact in the correct determination 

 of the age of these beds, and they all apparently agree in con- 

 clusions. 



Concerning the interpretation of the foregoing fossils, the fol- 

 lowing facts may be stated : The plants, as determined by Pro- 



S-BIOL. Soc. WASH., Vor,. VIII, 189:5 



