TROCHUS. 497 



terranean. Its presence in our seas is probably not due to 

 an occurrence in the bounds of its present limits, but rather 

 may have relation to its ancient existence in the region of 

 the crag. It is recorded by Mr. Searles Wood as a fossil 

 of the coralline crao- at Ramsholt. 



T. ALABASTRUM, Beck. 



Uniform white, or tinged with golden yellow upon the carina 

 only ; whorls of the spire with three strong spiral keels, the 

 body volution with four ; axis imperforated. 



Plate LXVI. fig. 7, 8, as T. formosus. 



Trochua quodridnctus (fossil undescribed) Annals Nat. Hist. 1842. 



„ occide7iluli$, MlGHELS and Adams, Boston Jour. Nat. Hist. (Nov. 1841 ) 

 vol. iv. p. 47, pi. 4, f. 16 probably. 

 Margarita alabastrum, Beck in Loven's Index Moll. Scand. (1846) p. 20 (from 



type). 

 Trochus formosus, Forbes, Annals Nat. Hist. vol. xix. (Jan. 1847) p. 96, pi. 9, 

 f. 1.— S. Wood, Crag Moll. p. 125, pi. 13, f. 2 (fossil). 



This extremely rare shell is wrought with a sculpture so 

 peculiar, as to enable us to distinguish even a fragment of 

 it from any of its British congeners. This consists of a 

 few very prominent, strong, but rather narrow, rounded, 

 but sharply defined, subequidistant carinee or spiral belts, 

 of which the top one is generally broken into tubercles ; 

 indeed, upon the upper volutions, two if not the whole 

 three — for such is the usual number, excepting upon the 

 body, which is generally adorned with four — are similarly 

 affected ; the interstices are smooth, and a little concave. 

 The general shape of the shell, which does not appear to 

 be particularly solid, nor yet translucent, and seems but 

 slightly polished (the examples taken are chiefly dead 

 specimens, so that we presume not to speak decidedly on 

 these points), is simply conical, with the lateral outlines 



vol. ii. 3 s 



