XVIII PEELIMINARY EEMARKS. 



stroys them, thereby discouraging persons S*om collecting. This, however, 

 might in a measure be remedied by judiciously cleaning the specimens from 

 unnecessary sand and marl when first taken from the pits, and after thorough 

 drying, soaking the specimens in boiling glue, of a consistency sufficient 

 readily to penetrate the marl, which, on rapidly drying again, will be har- 

 dened and protected so as to be preserved for an indefinite period. Speci- 

 mens which I treated in this manner twenty-six years ago are as perfect 

 to-day as when first prepared. 



It will be noticed that by far the larger part of the species here de- 

 scribed are from the Lower Marl Beds. Leaving out the BracMopods, of 

 which there are only five given, including one doubtfully from within the 

 State, there are two hundred and thirty species and varieties. Of these, five 

 are from the clays below the Lower Marl Bed; eleven, including Gryplicea 

 vesicitlariSj which occurs most abundantly in the Lower Marl Bed, are from 

 the Middle Marl Bed ; seventeen are from the Cretaceous layers at the base of 

 the Upper Marl Bed ; twenty-three are from the Eocene layers at the top of the 

 Upper Marl Bed ; and twelve species of Unionidce are from the clays at Fish 

 House. This leaves one hundred and sixty-three species for the Lower 

 Marl Bed, only one of which is known to occur in any of the other beds; 

 Gryphcea vesicularis being common to the Lower and the Middle Beds, and 

 also recognized in a single individual in the Eocene layers. Idonearca 

 vulgaris was supposed to pass from the Lower to the Middle Bed, but I find 

 that the Middle Marl Bed specimens belong to quite a distinct form. Ostrea 

 or Gryphcea Bryani Gabb, in one of its varieties, appears to be common to 

 the Middle Bed and to the lower layers of the Upper Bed. Beyond 

 these I have no evidence of a passage of species from one bed to any of 

 the others. There is, .however, some difficulty in satisfactorily determining 

 whether this is really the case, as most of the collections which I have ex- 

 amined are not carefully labeled with locality or position, and from their 

 great similarity of material they present a peculiar liability to error. This 

 may be said to be particularly the case with the collections -deposited in 

 the Academy of Natural Sciences in Philadelphia, many of which have no 

 other record of locality or position, either with the specimen or embodied 

 in the description of the species of which they are the types, than simply 



