On the Form, Growth, and Construction of Shells. 243 



as right. Tlie greater part of tlie genus Clausilia (numbering 

 upwards of two hundred species) is reversed. The species of 

 the genera Physa and Triforis appear to be all reversed. All 

 the specimens of Fusus contrarius, Sby., so abundant in the 

 Red Crag, and also found living in Yigo Bay, on the coast of 

 Spain, are left-handed. But, after all, these latter are the 

 exceptions. Every one familiar with garden plants will have 

 noticed that the hop turns round its pole in one direction, 

 going to meet the light, while the scarlet-runner takes an 

 opposite course, as invariably as the sun it follows.* 



The tendency to spirality is observable in bivalve shells, 

 especially in Isocardia, whose separate valves resemble two 

 spiral univalves — one right-handed, the other left-handed (see 

 Coloured Plate, Fig. 12), with small spires and large apertures. 



In this shell, as in the common cockle, one valve is placed 

 on each side of the shell-fish, which is usually symmetrical, and 

 lives in a vertical position as regards the plane of its valves. 



Fig. 2. Fig. 3. 



Fig. 2 represents the internal east of Diceras arietlnum, Lamk. Coral Rag, 

 France, \ nat. size, a, point of attachment ; c,c,c', casts of dental pits ; 

 t,t,t, t', t\ furrows produced by muscular ridges. 



Fig. 3, internal cast of Requienia Lonsdalei, Sby. sp., J nat. size, Lower Green- 

 sand, Bowood, Wilts. From Quart, Journ. Geol. Soc, 1855, Vol. XI. p. 53, 

 Figs. 28 and 29. 



To this there are some exceptions j for example, the oyster 

 and scallop, which, like the turbot and sole among fishes, lie, 

 the former on its left side, and the latter on its right side, and 

 fatten at their ease. 



The valves of the cockle are united by an elastic ligament, 

 and articulated by projecting teeth, which form a very complete 

 hinge. It is obvious that the valves of a shell, cannot grow so 

 freely along the hinge, as on the rest of the margin, but it 

 may shoot out to a great length, as in the " razor-shell " 

 (So len), or in three directions, as in the " hammer-oyster " 



* Unluckily the botanists have chosen to reverse the terms employed in 

 mechanics, and call the spiral of the hop right-handed. 



