PREFACE. 5 



accurate knowledge of the stratigraphy of the region. This 

 part of the report, however, is not confined to the Mollusca and 

 Brachiopoda as was Whitfield's monograph, but contains also, 

 descriptions of the other invertebrate groups. The chapter on 

 the Protozoans has been compiled from Bagg's work on the New 

 Jersey Cretaceous Foraminifera.i The chapter on the Echinoids 

 also is strictly a compilation from Clark's report on the Mesozoic 

 Echinodermata,^ to which has been added the descriptions of 

 several new forms, generously furnished by Dr. Clark. 



In comparing the faunas of these New Jersey Cretaceous beds 

 with similar faunas elsewhere, the condition of preservation of 

 the New Jersey specimens has been a serious obstacle. Many of 

 the New Jersey species are known only in the condition of in- 

 ternal casts while the species from the Southern States have 

 mostly been described from specimens preserving the shells. The 

 recent collections, however, have afforded many species with the 

 shell preserved and in addition to these the external impressions 

 of the shells have frequently been secured, from which plaster 

 casts or wax squeezes have been obtained which have given the 

 external characters of the shell. 



In many of the more or less incoherent beds of the region the 

 shells have been removed by solution, after which the cavities 

 left by them have been closed by pressure, the resultant speci- 

 mens being somewhat modified internal casts with the markings 

 of the exterior of the shell impressed upon them. At first it was 

 found to be difficult to preserve such material because of its 

 incoherent nature, the specimens crumbling more or less easily 

 on becoming thoroughly dry. This difficulty has been obviated, 

 however, by carefully cleaning the material while still moist, soon 

 after being removed from the ground, then, after being thor- 

 oughly dried, with very careful handling, the specimens have 

 been immersed in molten paraffin for from 40 minutes to one 

 hour. On cooling after removal from the paraffin, these speci- 

 mens have proved to be in excellent condition for permanent 

 preservation. Other specimens preserving the shell itself in a 



' Bulletin U. S. G. S. No. 88. 

 - Bulletin U. S. G. S. No. 97. 



