Logan.] The Invertebrates, Benton Group. 441 



FORT BENTON CRETACEOUS FAUNA. 



The most abundant forms among the invertebrate faunas 

 of the Fort Benton Cretaceous are Inoceramus labiatus and 

 Ostrea congesta. In the lower horizon, including the Lincoln 

 marble and its adjacent shale beds, Ostrea and Inoceramus pre- 

 dominate. The invertebrate forms in the Lincoln marble are 

 associated with sharks' teeth, the remains of fishes, turtles, 

 and plesiosaurs. Inoceramus labiatus is the most abundant form 

 in the Fort Benton limestone. One layer is composed almost 

 wholly of this shell, and resembles a mortar bed into which 

 these forms have been placed. Belemnitellse are also very 

 abundant in this horizon, but good specimens are difficult to 

 obtain. Prinonotropis and Prionocyclus are both well repre- 

 sented, but Ostresc are rare in the limestone group. In the 

 shale beds (Ostrea beds) overlying the limestone, Ostrea congesta 

 is the most abundant form ; it occurs either free or attached to 

 Inocerami. Sharks' teeth and pavement plates are also abun- 

 dant in the Ostrea beds. 



In the lower horizon of the Blue Hills shales Serpulse are 

 found attached to large Inocerami. The middle zone of these 

 shales seems to be devoid of fossil forms, but the upper horizon 

 in which the septaria nodules are imbedded has furnished many 

 invertebrate forms. Of these the Scaphites and Inocerami are 

 the most abundant. Nearly all the shells of this horizon have 

 the pearly layer preserved ; this does not occur in any other 

 Cretaceous horizon except in the Fort Pierre. Four of the sub- 

 kingdoms are represented in the Fort Benton, viz. : Coelenterata, 

 Echinodermata, Vermes, and Mollusca. The first three are 

 each represented by a single form. Mollusca, however, are rep- 

 resented by eighteen genera and forty species. I think it may 

 be truly said that more than one-half of these forty species oc- 

 cur in the Septaria zone. 



