MOLLUSC A. 157 



formed of shells. " The thin inner layers of some large flat 

 bivalves, when polished, are used in the south of China, and 

 in India, instead of glass, for windows."* Many of the do- 

 mestic utensils of uncivilised nations are shells ; and they are 

 converted into drinking-cups, knives, spoons, fishing-hooks, 

 and even razors. " In Zetland, one of our common univalve 

 shells (Fusus antiquus), suspended horizontally by a cord, is 

 used as a lamp, the canal serving to hold the wick, and the 

 cavity to contain the oil." In former times the scallop 

 (Pecten maximus, or opercularis) was worn by religious pil- 

 grims, a custom occasionally referred to by our poets. Thus, 

 Parnell says of his hermit, 



" He quits his cell, the pilgrim staff he bore, 

 And fixed the scallop in his hat before." 



The difference in point of size is not less remarkable than 

 that of the form and colouring. The Tridacna, or Giant 

 Clamp-shell (Fig. 150) is said to attain occasionally a weight 



Fig. 150. TRIDACNA. 



of more than 500 pounds; from which circumstance the story 

 may have originated of an oyster which furnished a dinner to 

 a whole regiment. Let us, in imagination, contrast with this 

 the microscopic chambered shells, of which Soldari collected 

 the astonishing number of 1 0,454, t from less than an ounce 

 and a half of stone found in the hills of Casciana, in Tuscany. 

 " Some idea of the diminutive size of these shells may be 



* From a scries of papers on Molluscous animals, signed " G. J." in 

 London's Magazine of Natural History. They are from the pen of 

 Dr. Johnston, author of the History of British Zoophytes, &c. who has, 

 In the kindest manner, authorised us to make use of them. 



f Dr. Buckland's Bridgewater Treatise, vol. i. page 1 1 7. 



