20 On the Form, Growth, and Construction of Shells. 



having three rows of spinous fringes produced at nearly 

 coincident intervals on each whorl of the shell, and becoming 

 longer with age. " Venus's comb/' Murex tenuispina, is an 

 instance of this, the canal of the shell being produced to twice 

 its length, and fringed with three rows of long and slender 

 spines, slightly curved, like the teeth of a harrow. In the 

 Coloured Plate we give a less common example, the Murex 

 adustus (Coloured Plate, Yol. x., p. 241, Fig. 7), the spines 

 of which are extremely picturesque, reminding one of the 

 branching fir-tree. 



In the " Wentle-trap," Scalaria pretiosa (see Plate, p. 

 21, Fig. 2), the periodic mouths encircle the shell-whirls, 

 which are sometimes separate, and contribute not a little to 

 the beauty of this once costly conchological treasure. 



Just as with the growth, we see periodic markings on the 

 external surface of the shell, so also in the section of almost 

 any spiral shell we see a repetition of folds around the 

 columella, or internal pillar of the shell, and sometimes upon 

 the sides of the whorls (see Plate, Vol. x., p. 245, Figs. 6 and 8). 



But the most marked character in adult univalves is pro- 

 duced by the formation of a final aperture and ultimate lip to 

 their shells. This is well seen in the f< spider/* or " scorpion- 

 shell/' Pteroceras, from China, in which the apex of the shell 

 is concealed in a long, claw-like spine, while six others extend 

 from the outer lip, and the canal is curved to correspond with 

 the apical spine. 



In the great fossil Rostellaria ample/,, from the middle 

 eocene formation of Barton, Hants, the adult animal puts forth 

 a widely expanded lip, as broad as one's hand. In Vermetus 

 (Fig. 9, Coloured Plate, Vol. x., p. 241) and Siliquaria (Fig. 

 10) the whorls become disunited in age. In Aspergillum (Fig. 

 13), each periodic growth is marked by an additional frill to its 

 siphonal tube ; and when adult, it forms the curious perforate 

 disk from which it obtains its name. 



The " Cowry" (Cyjprcea), so common an ornament upon the 

 mantel-piece, when young, is a thin spiral shell ; but when it 

 becomes adult, it thickens its aperture enormously by repeated 

 depositions of shell-matter, until we fail to discern the apex at 

 all. In the Plate, Vol. x., p. 245, Fig. 7, is represented a 

 transverse section of Cypraza turdus, which well illustrates this 

 peculiarity. See also figure of Oypro? a guttata (Plate, ~p . 21, Fig. 4; 

 drawn from a specimen in the British Museum valued at £40) . 



But perhaps the most marked change which takes placo in 

 adult shells is to be seen in certain land-snails belonging to 

 the Helicidaz. In Gibbus Lyonetti, from Mauritius, the shell, 

 after forming five ordinary convolutions, suddenly makes a 

 complete double in its growth, and remains hump-backed for 



