PEEFACB Vll . 



As the proposed object was to exhibit all that was known of 

 a local fauna, many species are described from more or 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 obj ects 

 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 inclu- 

 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 ? variety 

 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 ; lat. from the 

 anterior to the posterior ends ; alt. the thickness of the closed 

 valves. In the spiral univalves ; long, from the vertex to the 

 base ; long. spir. from the vertex to the posterior end of the 

 labrum ; lat. 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- 



