380 SCALARIA.VERMETUS. 



the Periwinkles ascend the rocks, it is a sure sign of a storm 

 being at hand, their instinct having taught them to place them- 

 selves out of the reach of the dashing of the waves ; when, on 

 the contrary, they make a descent upon the sand, it is an indica- 

 tion of a calm. The true Turritellce have an elongated spiral 

 shell, with a small aperture ; and nearly allied to them is the 

 Scalaria, in which the surface of the convex whorls is adorned 

 with numerous, very prominent, transverse ribs. In some cases 

 the whorls are separate, and only come in contact by their ribs. 

 The principal species of this group the Scalaria pretiosa, or 

 Wentle-trap was long famous on account of the high price 

 given for it by shell-collectors. 



995. Besides these normally constructed shells, some of the 

 Mollusks of this family form an irregularly tubular habitation, 

 which so much resembles that formed by certain Annelida, as to 

 be scarcely distinguishable from it. They are very few in 

 number. The Vermetus (Fig. 672) is the principal genus of the 

 group. This is remarkable for the close resemblance of its shell 



FIG. f.T2. VEUMKTUS. 



to that of the Serpula (Fig. 629) ; but, when perfect, it may be 

 generally distinguished by the regularly-spiral twisting of its 

 first-formed portion. Some species associate together in large 

 masses, so as even to form reefs ; whilst others attach themselves 

 to Coral, and lengthen their shells in proportion as the Coral 

 grows up around them. When the animal has quitted the lower 

 part of its tube, that it may keep itself at the surface, it usually 

 throws a partition across, in the manner of the Nautilus ; and a 

 tolerably regular series of such partitions is not unfrequently 

 found. In the Magilus, an allied genus of similar habits, whose 

 tube is sometimes lengthened in this manner to as much as three 

 feet, the first-formed part of the shell, instead of being cut off, 



