UNIVALVE SHELLS. 479 



When a shell is placed on its base, and the spire points 

 directly upwards, the spire is erect ; but if it points slanting 

 heavenward, it is oblique. 



If the spire is abruptly terminated by the loss of its upper- 

 most whorls, it is said to be decollated. Few shells exhibit 

 this remarkable character. Bulimus decollatus is the most 

 familiar example. Were it to have the spire unbroken, 

 there would be fourteen or fifteen whorls, but there are only 

 six or seven. When the " Barnet" of Adanson (which is a 

 marine shell) has formed eleven whorls, the upper ones fall 

 off, and only four or five remain ; and the "Popel," a species 

 of Cerithium, exhibits the same phenomenon,* The process 

 by which it is effected has been explained in a previous 

 letter. See also Encyclop. Method, v. i. 327. Mr. Stutch- 

 bury has seen the Bulimus decollatus forcibly strike the 

 apex of the shell against a stone, for the purpose of decol- 

 lating itself.-f- 



The whorls usually overlap at the line of junction and are 

 as it were soldered together, but in a few rare examples they 

 are separate or detached. The best example of this struc- 

 ture is exhibited in the Wentletrap (Scalaria pretiosa), which 

 has obtained a certain notoriety in the history of Conch- 

 ology from the large prices which have been given for perfect 

 specimens. " In 1753, at the sale of Commodore Lisle's 

 shells at Longford's, four Wentletraps were sold for 751. \2s., 

 viz.: First day, Feb. 21, lot 96, one not quite perfect, 

 161. 16s. Third day, lot 98, a very fine and perfect one, 

 18/. 18s. Fourth day, lot 101, one for 16/. 16s. Sixth day, 

 lot 83, one for 231. 2s." % 



The line which marks and defines the union of the whorls 

 is called the suture. It is either plain, or channeled, or raised. 



The surface of the whorls may be plane, or convex, or an- 

 gnlated. It is very variously sculptured with striae, grooves, 

 ridges, murications, spines, ribs, and spinous processes. 

 When these run, or are arranged, in the same direction as 

 the whorls, they are said to be spirally drawn or rowed ; and 



with advantage to the lovers of rarities. When a garden snail is placed with 

 its apex vertical, its aperture expands ordinarily to the left. The line 

 of curvature is swelling towards the right. But varieties occur, though 

 rarely. The rarity gives them value to collectors. The Frenchman ob- 

 tained a living pair, and produced a fine family, all of whom, from their very 

 birth, went the wrong way ; all inclining to the cote gauche, — revolutionists 

 from the egg.'' — Duncan's Analogies of Organized Beings, p. 121. 



* Senegal, 147, 153. Also Lin. Syst. 122G. 



t Carpenter's Gen. and Comp. Physiology, 97. 



X Da Costa's Elements, 204. 



