Maryland Geological Survey 53 



The Monmouth fauna is much the most prolific of any of the Upper 

 Cretaceous faunas of Maryland; 158 or possibly 164 forms have been 

 specifically determined, and there are a number of other species, most of 

 them new, which have been disregarded because they are too poorly pre- 

 served to serve as types. Over 80 per cent of this fauna is peculiar. As in 

 the Matawan fauna, the percentage of restricted pelecypods is much lower 

 than that of either the gastropods or cephalopods ; only a little more than 

 70 per cent of the Monmouth bivalves are peculiar to the horizon, while 

 about 94 per cent of the univalves and all three of the cephalopods are 

 restricted to that formation. Not more than 3 or possibly 4 of the 164 

 species run down into the Magothy, although about 22 per cent of the 

 Monmouth forms occur in the Matawan. The Monmouth and Eancocas 

 have only 5 species in common, 3 of the 5 being wide-ranging forms which 

 are initiated before the opening of the Monmouth. The other 2 occur 

 only in the Monmouth and Rancocas. 



The Eancocas fauna is only imperfectly known. Gastropods undoubt- 

 edly are present, but none were found in a determinable state, so that all 

 of the 8 species recorded are bivalves. Out of the 8, 3 are restricted in 

 their known distribution to the Rancocas, 2 to the Monmouth and Ran- 

 cocas, 2 to the Matawan, Monmouth and Rancocas, and 1 extends down- 

 ward as far as the Magothy. Gryphcea vomer has not been reported from 

 the Rancocas of Delaware, although it occurs at a similar horizon in New 

 Jersey. It is the only Upper Cretaceous mollusk of Maryland which is 

 known to survive the break between the Mesozoic and the Cenozoic. 



From a biologic point of view the most interesting feature of the fauna 

 is the relatively large number of Prionodesmacea, 75 out of the 129 

 bivalves, almost 53 per cent, being included in the most primitive of the 

 three pelecypod orders. In the succeeding Eocene of Maryland only 24 

 out of 55, or 44 per cent, are referable to the Prionodesmacea, and in 

 the Miocene of Maryland only 53 out of 187, or 28 per cent of the entire 

 number. 



A few of the genera represented, notably Perissonota, Nemodon and 

 Paranomia, all of them described by Conrad from East Coast species, 

 have not been recognized excepting from the Upper Cretaceous. Inoce- 



