Age of the Trinity Division. 



17 



Viviparns natica (?) cossatotensis Hill. 

 Gylindrites (?) sj). indet. 

 Baccinopsis (?) parryi Conrad. 

 Tylosloina pcdernalis (Rociner). 

 Vicarya hr<mneri sp. nov. 

 Nerinwa austinensis Rocmcr. 

 Neritina sp. indet. 

 NcAimayria ivalcotti Hill. 

 Acanthoccras (?) jiistin,T sp. nov. 



Arthropoda : 



Cypridca te.mvn sj). nov. 



Vertebrata : 



Lcpidofus. 

 Crocodila. 

 Dinosaiiriin. 

 Chclonia. 



Plantar : 



Many species, now in course of pul;)lication by Professors 

 Ward and Fontaine, to<;etber witb undetermined species 

 resemliling Aran car ites, figured on plate 1. 



IV.— AGE AND SIGNIFICANCE OF THE TRINITY DIVISION. 



Witb tbe exception of tbe genera Reqidenia and Monopleura, 

 tlie above list contains none of tbe bitberto familiar types of fos- 

 sils found in tbe overlying Fredericksburg and Wasbita Divisions, 

 sucb as tbe cbaracteristic Echinodermata, Radiolites (alleged IIij> 

 piirites), Gryplixas of tbe Fitchcri group, Exogyras, or Schloenbach- 

 iate Ammunitidas, but possesses a molluscan fauna peculiarly its 

 own, so fiir as America is concerned, wbicb, accompanied by a 

 well-|)reserved flora and vertel)rate fauna, affords tbe first satis- 

 I'actory and complete data for an age classification of tbe sul)- 

 divisions of tbe Nortli American Lower Cretaceous formation, 

 and will enal)lc us to more tborougbly interpret tbe succeeding 

 divisions. Tiiis association of vertebrates, plants, and marine 

 mollusca is a most important fact in tbe correct determination 

 of tbe age of tbese l)eds, and tbey all apparently agree in con- 

 clusions. 



Concerning tbe interpretation of tbe foregoing fossils, tbe fol- 

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



3-BiOL. Soc. Wash., Vol. VIH, IS'J.i 



