PREFACE v. 
only account of the shells of Mazatlan previously known, is 
Dr. Menke’s list of the species brought by Mr. H. Melchers, 
published in the Zeitschrift fiir Malacozoologie, 1847-51. An 
analysis of these is given in the Brit. Assoc. Rep. pp. 235-239. 
The species of Bryozoa,* (now first, I believe, included in a 
catalogue of Mollusca, although generally acknowledged by 
naturalists to belong to that Subkingdom) have been described 
by G. Busk, Esq. with his usual kindness. The class is named 
as in Dr. W. B. Carpenter’s “Principles of Comparative Physi- 
ology, 1854.” The name Polyzoa is believed to have precedence: 
but while the names of genera and species are proper names, 
and therefore ought to follow the law of priority, the arrange- 
ment of classes and orders is a matter of opinion ; and it appears 
allowable to make use of those names, whenever given, which 
best express the leading characteristics of the division. For 
this reason Palliobranchiata and Lamellibranchiata are used 
instead of the older names Conchifera and (or rather, including) 
Brachiopoda ; Proboscidifera for Zoophaga, &c. In the pre- 
sent case, when the name of a supposed order (of Polypes) 
becomes entitled to rank as a Class (of Molluscs), and as such 
has to be learned in common schools, it appeared very important 
to select a name that could not easily be confounded with 
others of similar sound: 
In the bivalves, the order of Prof. E. Forbes, adopted by 
Mr. Woodward in his invaluable ‘‘ Manual of the Mollusca,” 
has been mainly followed : in the univalves, that of Dr. Gray, 
who obligingly lent me the proof sheets of his “Systematic 
Arrangement of the Mollusca” now passing through the press. 
I am not possessed of sufficient knowledge of physiological 
anatomy to give an independent opinion on disputed points. 
Having found considerable difficulty in the identification of 
species, when Lamarckian genera are retained without division, 
although now as numerous and diversified in recorded forms 
as were many of the Linnean genera in the days of Lamarck, 
I have freely adopted many of the generic names recently pro- 
posed, and have even, in some few cases, addedtothem. Itisa 
matter of secondary importance, whether an accurately defined 
group takes rank as a mere section in a subgenus, or as a lead- 
ing division in a family: but the binomial designation is much 
easier for reference than that by sections. For ordinary purposes 
* The Crustacea, Cirripedes, Annelids, Radiata, &c. found in the Reigen 
Collection, are deposited in the Warrington Museum ; which has the honour of 
beimg the first Free Museum and Library established in the manufacturing dis- 
tricts. Duplicate series of the shells can be obtained through the Curator. 
