PREFACE Vil. 
As the proposed object was to exhibit all that was known of 
a local fauna, many species are described from more cr less 
imperfect materials, which would not have been noticed if 
from a mixed collection. The same course is usually followed 
in describing the fossils of any given formation, where objects 
are carefully noted that a mere collector of “good shells” 
would cast-aside as worthless. There appears no reason for 
denying all knowledge of existing forms, merely because that 
knowledge is not as full as may be desired. 
In spite of the interests of classical Latinity being freely 
sacrificed, whenever those of brevity come into collision, the 
diagnoses of species will generally be regarded as much too long. 
The reasons are, (1) that I generally had to describe not single 
specimens, of which a literary picture could be drawn, but a 
large number, all whose observed differences had to be inelu- 
ded: and (2) that, in our present state of very limited know- 
ledge, it is necessary so to describe as not only to separate the 
object from previously known species. but from other similar 
ones that may be hereafter discovered. Those who have en- 
deavoured to identify critical species from the descriptions of 
some of the early naturalists, would willingly part with the 
acknowledged convenience of brief diagnoses for the sake of a 
greater approach to accuracy. When a fauna (like the British) 
has been well explored, a careful analysis of species may allow 
of their identification with but few words of separative descrip- 
tion. In many instances, the materials at command were not 
sufficiently clear to decide whether differences of form were 
of specific or only of varietal value. In these cases, they are 
generally tabulated as conspecific; but with the Pvariety 
separately named and described, ready afterwards to take rank 
as a species, or to merge into the related form, as further facts 
or better judgment may decide. 
The measurements (unless otherwise expressed) are, in obedi- 
ence to an authority, thus taken, In bivalves; long. from the 
umbo to the middle of the ventral margin; Jaé¢. from the 
anterior to the posterior ends; a/é. the thickness of the closed 
valves. In the spiral univalves; Jong. from the vertex to the 
base ; long. spir. from the vertex to the posterior end of the 
labrum ; dat. the diameter of the body whirl; div. the mean 
angle of divergence of the spire outlines. All the measure- 
ments of length are given in inches and decimal portions. 
In describing sculpture, (the words longitudinal and trans- 
verse not havingbeen always used in the same sense) the follow- 
ing terms have been generally employed. In the bivalves ; con- 
