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



which are distinct from the outer, and from each other. The 

 cavities thns formed are found to contain water, which, how- 

 ever, evaporates after the specimens have been placed in a dry 

 situation for a long period : but the water is again absorbed by 

 immersing the specimens for a sufficient number of hours. This 

 reduction of the inner space appears to be effected in order to 

 counteract the continued increment of the shell (by deposits of 

 new shell-matter, along its margin from the border of the 

 mantle) at a greater rate than is required for the accommo- 

 dation of the soft parts of the animal. 



The tubes of Vermetus (see Coloured Plate, Fig. 9), and 

 Magilus, and the apices of Triton, Turritella, and Euomphalus 

 become either partitioned off or filled up solid, in the continued 

 growth of the animal. 



Shells owe their variations in form to a number of circum- 

 stances. Those which assume a spiral, vary in being either 

 turbinated or discoidal in their growth, and again, in the infi- 

 nite gradations between the extremes of these two. The shape 

 of Conns (see Plate, p. 245, Fig. 4), is an inverted triangle, that 

 of the telescope shell (Cerithium telescopium, Fig. 8) trapezoidal, 

 and so on. The turbinated shells again merge into forms in which 

 the whorls become detached with age as in Vermetus (Coloured 

 Plate, Fig. 9), and Siliquaria (Fig. 10), or a nearly straight tube 

 like Dentalmm (Fig. 8). The discoidal shells graduate into 

 forms having fewer and fewer convolutions, and wider and 

 simpler mouths, until at last, in forms like Patella (Coloured 

 Plate, Fig. 6), all spirality is lost, and we have only a tent- 

 shaped cover. 



DESCRIPTION OF COLOURED PLATE. 



Fig. 1, Cleodora pyramidalis, L., Atlantic; Fig. 2, Bidimus 

 regina, Fer., Amazon ; Fig. 3, Trojndophora carinata, Mau- 

 ritius ; Fig. 4, Helix {Anastoma) globulosa, Lam., Brazil ; 

 Fig. 5, ditto (under side); Fig. 6, Patella longicosta, Lamk., 

 West Indies ; Fig. 7, Murex adustus, Lam. ; Fig. 8, Dentalmm 

 elephantinnm, Red Sea; Fig. 9, Vermetus lumbricalis, Gin. sp., 

 West Africa ; Fig. 10, Siliquaria anguina, Linn., New 

 Guinea; Fig. 11, Waldheimia australis, Quoy, Port Jackson; 

 Fig. 12, Isocardia cor, Lin., British; Fig. 13, Aspergillum 

 vaginiferum., Lam., Eed Sea; Fig. 14, Chiton squamosus, \ Aim., 

 West Indies. 



DESCRIPTION OP PLATE (PAGE 245) — OPERCULA AND SECTIONS OF 

 SPIRAL SHELLS. 



Fig. 1, Operculum of Turbo Jhtctuaius, Mazatlan — a, ex- 

 ternal surface ; Fig. 2, Operculum of Imperator olivaceus, 



