iv.— VJlL PEEFACE 



the Tertex to the posterior end of the labrum ; lat. the diame- 

 ter of the body whirl ; div. the mean angle of divergence of 

 the spire outlines. AH the measurements of length are given 

 in inches and decimal portions." 



"In describing the sculpture of bivalves, concentric s\gm&e3, 

 in the direction of layers of growth ; radiating, from the 

 umbo to the margin. In the spiral tinivalves; radiating 

 (from the axis of the sheU), in the direction of layers of growth ; 

 .spiral, along the whirl, parallel to the suture. In comparing 

 the words used to describe sculpture (lirfe, lirulae, sti-ise, striulse, 

 &c.) 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 difierent authors 

 for the same shell, may sometimes vary."* 



"The terms expressing frequency are generally used as 

 follows : extremely rare, under a score ; very rare, under 100 : 

 rare, under 200; not common or not uncommon, 'BOO ; com man, 

 up to 400 or 500; abundant, about 600 or 700: extremely 

 common, up to 1,000 ; extremely abundant, more than 1,000." 



"In citing geographical authorites, the name of the first 

 observer is distinguished by Italics. 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 Eeigen Collection as a 

 geographical authority, and a comparison of it with other 

 neighbouring faunas, will be found in the 'Keport 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. 

 185G, and published in its Transactions, pp. 159—368." 



JOHN EDWARD GEAY. 



British Museum : April22)id, 1857. 



•"This may account for discrepancies in VitrineUa, and in the descriptions 

 of the nuclea,- vertox m Pyramidellidte. In this Catalogue, the number w OTer 

 rather than uudcr-ratcd." 



