LAMELLIBRANOHIATA OF THE PLASTIC CLAYS. 21 



material with which I have had to work. The great difficulty of obtaining 

 collections has been the greatest drawback with which I have had to con- 

 tend. The specimens are mostly internal casts poorly preserved in a soft 

 friable material, which is strongly impregnated with pyrite, so that they 

 readily decompose and crumble, making it impossible to preserve them for 

 any length of time. For this reason, collections made a few years ago of 

 apparently good fossils, are represented at present only by a few shapeless 

 masses of marl and a quantity of loose sand; and I present this as an ex- 

 cuse for illustrating such imperfect material as will be found on the plates. 

 If collectors would take their marl specimens when first collected, and after 

 cleaning and shaping them as desired, soak them carefully and thoroughly in 

 boihng glue-water, then dry them rapidly, they might be preserved in good 

 condition for a very long time. But this is seldom done. 



