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



(Malleus), while in the "heart-cockle" each valve takes a 

 spiral (see Coloured Plate, Fig. 12). 



There are some fossil shells called Diceras, in which the two 

 valves resemble horns, and others called Requienia (see Wood- 

 cut, Figs. 2 and 3), with one valve produced into a horn. In 

 Ghama the umbones are also spiral. 



In many bivalves the beaks are turned forwards, towards 

 the head of the animal (not always, as stated in some big 

 books). But the oyster is again an exception in this respect; 

 and if among the exotic or fossil species you find some with 

 spiral growth, the spiral is turned bachwards ; indeed, the 

 number of exceptions is so great, that one fears to make any 

 general assertion. Anomia, when it grows inside other shells, 

 may have its umbo a little removed from the margin, and the 

 fossil genera of Hippurites exhibit every condition between a 

 marginal ligament and a spiral beak, like Ghama, and a 

 patelliform valve, with a ligament wholly internal, and a central 

 umbo. 



The shell, considered as a defence, is most complete in 

 those bivalves, like the oyster, which shut up close (see Coloured 

 Plate, Figs. 11 and 12), and in those univalves which have an 

 operculum, or door, to their houses (same Plate, Fig. 3). 



Many bivalves gape a little at the sides (or ends), where the 

 foot and the respiratory tubes are accustomed to be pushed 

 out j and many univalves are too large and bulky, in proportion 

 to their shells, to be completely housed and protected in them. 



The cowry owes the glassy polish of its whole exterior to 

 the amplitude of its mantle, whose folds meet over its back, 

 and ordinarily conceal the shell entirely. 



In the shining Marginellas and Olives, and some Volutes, 

 the shell is partially glazed by the same envelope. 



One sort of garden-slug, the Testacella (see Woodcut, 



FiG. 4. Testacella haliotidea, side view; s, shell ; b, end view of a Bpecimen 

 which has been disturbed from its winter sleep, showing the mantle partly 

 expanded round the shell. 



Fig. 4), has a smal], shield-like shell on its tail, while the rest 

 appear to have no shell at all, it being always rudimentary, and 

 usually internal. Such is also the case in Aptysia, the "sea- 

 hare," in which the shell is entirely concealed by the mantle. 

 On the other hand, Gydostoma elegans (see Woodcut, Fig. 6), 

 a snail found in hedgerows and thickets of the chalk districts, 

 covers the aperture of its shell with a close-fittiug, shelly plate, 



