PALEONTOLOGY. 137 



presence of organic matter, the decomposition of which 

 has resulted in their production. Those containing 

 phosphate of lime or other materials of commercial 

 value are often extensively worked by pits or trenches ; 

 admirable opportunities are thus afforded for collecting 

 the fauna they exhibit, both in the pits themselves and 

 among the washed heaps of these products.* 



For the same reason all concretionary nodules, 

 whether accumulated in layers or not, should always 

 be examined and broken open, since many excellent 

 fossils are frequently obtained therefrom ; such as those 

 enclosed in the ironstone nodules of the Carboniferous 

 series, and in the Septarice of many clays. 



II. Examples. In illustration of the previous re- 

 marks let us refer to the diagrams, figs. 5, 6, 7, and 8, 

 and the descriptions contained in Parts I. and II. of 

 this work. 



(a.) It will be remembered that the first quarry we 

 entered, in imagination, was a sand-pit near the church, 

 in fig. 5 ; in the light grey sand at the bottom of this 

 pit a few fossils were noticed ; these should be^ex- 

 tracted from the matrix or detached with portions of 

 the rock, some being in the state of buff-coloured casts, 

 others retaining their shelly covering, some are frag- 

 mentary, others in a fairly perfect state. After a dili- 

 gent search among these friable beds we collect some 

 twenty or thirty specimens, which we recognise to be 

 various species of Ammonites, Terebratula, Ehynconella, 

 Ostrea, and Plicatula, &c., &c. These are carefully 



* The so-called "coprolite-pits" in the neighbourhood of Cam- 

 bridge form an example of this mode of occurrence, and the con- 

 sequent method of collection. 



