iv. PREFACE 
the vertex to the posterior end of the labrum; Jat. the diame- 
ter of the body whirl; div. the mean angle of divergence 
of the spire outlines. All the measurements of length are 
given in inches and decimal portions.” 
“Tn describing the sculpture of bivalves, concentric signifies, 
in the direction of layers of growth; radiating, from the 
umbo to the margin. In the spiral univalves; radiating 
(from the axis of the shell), in the direction of layers of growth; 
spiral, along the whirl, parallel to the suture. In comparing 
the words used to describe sculpture (lire, lirule, striz, striule, 
&e.) with the same words in other books, they should often be 
interpreted as to the appearance of the shell under the micro- 
scope, generally with an inch-achromatic. The vertex applies to 
the whole nuclear portion ; but the apex only to the first whirl. 
As a slight twist in this may, or may not, be estimated as a 
whole turn, the number of whirls, as stated by different authors 
for the same shell, may sometimes vary.’* 
“The terms expressing frequency are generally used as 
follows : extremely rare, under ascore ; very rare, under 100 ; 
rare, under 200; not common, or not uncommon, 300; common, 
up to 400 or 500; abundant, about 600 or 700; extremely 
common, up to 1,000; extremely abundant, more than 1,000.” 
“Tn citing geographical authorities, the name of the first 
observer is distinguished by Jtalics. The mark of uncertainty 
[?], is to be understood as always referring to what follows, 
and not to the word going before.” 
“An estimate of the value of the Reigen Collection asa 
geographical authority, and a comparison of it with other 
neighbouring faunas, will be found in the ‘Report of the pre- 
sent state of our knowledge of the Mollusca of the West Coast 
of N. America’ presented to the British Association in Sept. 
1856, and published in its transactions, pp. 159—368.” 
JOHN EDWARD GRAY. 
British Museum: April 22nd, 1857. 
* «This may account for discrepancies in Vitrinella, and in the descriptions 
of the nuclear vertex in Pyramidellide. In this Catalogue, the number is over 
rather than under-rated.” 
