_ FOSSIL BIVALVE SHELLS.” 381 
thousand ; and almost every day is adding new species, for 
scarcely a vessel arrives from distant seas without enriching 
the stores of the conchologist. ‘The numerous genera into 
which they are divided by systematists, and the constant 
_ changes effected in arrangement and nomenclature by every 
_ writer on the subject, render it difficult if not impossible to 
_ present the reader with any satisfactory epitome of modern 
_ conchology. 
I must restrict myself to a brief account of some of the 
- most common genera that occur in the British strata ; and 
shall dwell more particularly on those species which prevail , 
_ in the secondary formations, because they present the most 
important deviations from the recent types that are familiar 
to the general observer ; by this means, and by reference 
- to figures in standard works, the collector will, I trust, 
_ be enabled to identify the fossil shells which may most 
_ frequently come under his notice in the course of his geolo- 
gical rambles. 
FOSSIL BIVALVE SHELLS ; INCLUDING THE BRACHIOPODA AND 
LAMELLIBRANCHIA. 
Although in the modern Tertiary strata, as the Crag, and 
in the arenaceous beds of the Eocene formations, shells are 
generally found in so perfect a state, that no caution or 
knowledge is requisite for their collection, yet a few prelimi- 
_ nary remarks are necessary to point out certain conditions 
— in which the remains of mollusca, or evidence of their exist- 
_ ence, occur in the mineral kingdom, and particularly in the 
older fossiliferous rocks. Shells are found in the strata in 
_ the three following states :— 
_  Istly. Shells in which the constituent substance has un- 
 dergone but little change. Many of the specimens in the 
