On the Form, Growth, and Construction of Shells. 23 



the rest of its days. We have figured in our Coloured Plate, Yol. 

 x.,p. 241, the upper and under view of a still more eccentric land- 

 snail, the Helix (Anastoma) globulus (Figs. 4 and 5), from Brazil. 

 This snail, after growing like an ordinary Helix hortensis, or 

 arbustorum, suddenly pulls up, and, twisting his mouth up. 

 tight, produces the aperture on a plane with the spire ! 



Many of these land snails not furnished with opercula 

 fortify the entrance to their shell by secreting a number of shelly 

 plates, or teeth, around the aperture (see Coloured Plate, Vol. x., 

 p. 241, Fig. 4), so as to lead one to marvel how the occupant 

 of the shell managed to get in or out of his own house, and still 

 more how the eggs were excluded. In operculated Gasteropoda 

 (snails with a door to their houses), the growth of the shell 

 necessitates a corresponding enlargement of the lid, or oper- 

 culum; this constantly receives fresh shelly layers from the 

 mantle of the animal, and in those snails with a closely-fitting 

 operculum the growth is always in proportion to that of the 

 shell. Snails having spiral opercula (see Coloured Plate, p. 241, 

 Fig. 3, and Plate, p. 245, Vol. x., Figs. 1 and 3) rotate their oper- 

 culum slowly as they grow, so that the addition always takes 

 place along that portion of the margin which is next the 

 columella, or axis of the shell. The interior and exterior 

 surface of shelly opercula are very diverse, as seen in Figs. 1, 

 2, and 3 of the Plate, Yol. x., p. 245. 



It is absolutely essential that the mantle should cover any 

 part of the shell to which additions are required ; any injury 

 therefore beyond the reach of the mantle externally, must be 

 repaired from the interior. This will explain why the broken 

 apices of univalves, and the eroded umboes of the river-mussel, 

 are never repaired externally, but always by deposits within 

 the spire or the valves of the shell. 



In the Coloured Plate which accompanied the earlier pages 

 of this article, in November last, p. 241, we gave a number 

 of illustrations to show the extreme variation in the form and 

 growth of shells. In the plate which accompanies the present 

 part, at p. 21, we offer an additional series, in which we not 

 only notice variations in form, due to growth, etc., but also a 

 great variation in ornamentation resulting from colour. There 

 are few shells more persistent in form than are the " Cowries " 

 (Oyprcea) and Cones (Figs. 1, 4, 5, and 6) ; but the former 

 numbers upwards of 150, and the latter more than 200 species, 

 chiefly distinguished by the diversity of their colouration. The 

 Mitras (Figs. 7 and 8) are even more numerous than the 

 Cones (having some 400 species), many of which are striking 

 illustrations of brilliancy of colour. 



The Yolutes (Figs. 9 — 11), though a less numerous generic 

 division, contain nevertheless, many shells remarkable for 



