526 trochid^;. 



This very aberrant form of Trochus, differs so much 

 from the rest of its British congeners, as to be readily dis- 

 tinguished by the veriest tyro in Conchology. It is strong, 

 solid, opaque, with a coarse surface, yet smooth both 

 above and below, except in the young, which is obscurely 

 sulcated or closely girt with depressed obsolete costelloc, 

 vestiges of which are frequently visible upon the smaller 

 volutions of the adult shell. The shape is conoid, or orbi- 

 cular conoid, and the lateral outlines are more or less 

 convex. The exterior is, for the most part, of a yellowish 

 ash or pale fawn-colour, subreticulately variegated with 

 more or less crowded zigzag lines of a purplish black, 

 whose angles are small and frequent. Slightly eroded 

 examples, and these are the more common, exhibit an 

 almost uniform tint of brownish black. The volutions, 

 which terminate in a small, but generally abraded, blunt 

 apex, are about six in number ; they quickly enlarge, for 

 the last whorl is longer than the whole spire, are high, 

 the length of the penult being at the least equal to one 

 half its breadth at the superior suture, and are decidedly 

 rounded or even tumid, although just below the delicate 

 but well-marked sutural line they are a little retuse. The 

 base of the shell is rounded, yet a little compressed, espe- 

 cially upon the broad semicircular patch of white or 

 ochraceous orange that lies behind the pillar-lip, nearly in 

 the centre of the disk ; this appears as if worn flat, and is 

 occasionally bordered with bluish green. The basal cir- 

 cumference is not angulated ; the axis is imperforated, but 

 often exhibits, particularly in the more aged individuals, 

 an indentation which resembles an incipient umbilicus. 

 The mouth, which is very large, since even in mature indi- 

 viduals, it occupies nearly one half of the entire length of 

 the shell, and in the young a still larger proportion of it, 



