612 



of jtahtral 



and that one essential advantage of this pro- 

 ceeding consists in its protecting the lambs 

 from the sheep bot fly, and consequently 

 their larvae, if it is done early enough. 



SHELDRAKE, or SHIELDRAKE. (Ta- 

 dorna vulpanser.) An elegant species of 

 Duck, belonging to the genus Tadorna, up- 

 wards of two feet in length, which frequents 

 many parts of our coast, and remains through- 

 out the year. The head and neck of the male 

 is of a dark green ; lower part of the neck, 

 coverts of the wings, back, sides, rump, and 

 base of the tail, pure white ; the scapulars, ab- 

 domen, quills, a large band across the belly, 

 and tips of the tail feathers, deep black. A 

 large bay-coloured gorget adorns the breast, 

 and the wing is ornamented with a spot of 



SHELDRAKE. (TADORNA VULPAN3ER.') 



purple-green. The bill, and the fleshy knob 

 at its base, deep red. Feet, flesh-colour. It 

 may often be seen about our largest rivers. 

 Its food consists of small testaceous mol- 

 lusca, small fish, small Crustacea, and manne 

 plants. The female commonly selects a 

 rabbit-hole in which to deposit her eggs ; 

 these are ten or twelve in number, and of a 

 pure white : when there is not the con- 

 venience of a burrow for nidification, she 

 resorts to the fissures and cavities of i ocks. 

 Sheldrakes are very abundant in Holland 

 and on the coasts of France. They may be 

 domesticated, and are handsome ornaments 

 in poultry-yards ; but their flesh is rank 

 and bad. 



SHELL. The hard calcareous substance 

 which either protects the testaceous Mollusca 

 externally, or supports certain species of 

 them internally. It has been truly said, 

 that he who would know the nature of Shells, 

 must know first the nature of the animals 

 of which Shells form a part ; and to this end 

 we at once refer the reader to the article 

 MOLLUSCA. Although Shells, properly so 

 called, which form the habitation of testa- 

 ceous animals, are sometimes confounded 

 with the shelly coverings which protect the 

 Crustacea (Crabs and theii numerous allies), 

 a very obvious and striking difference exists 

 between them, as well as Between the kinds 

 of animals which respectively inhabit them. 

 The Shells of Testacea are composed of car- 

 bonate of lime, combined with a small por- 

 tion of gelatinous matter : they are, in ge- 

 neral, permanent coverings for their inha- 



bitants ; and the animal is of a soft substance, 

 without bones of any kind, and attached to 

 its domicile by a certain adhesive property. 

 On the other hand, those animals which are 

 defended by a crustaceous covering cast their 

 shells, and renew them annually ; while the 

 animals themselves are of a fibrous texture, 

 with articulated limbs, and protected, as it 

 were, by a coat of mail. 



Shells are divided into Multivalves, Bi- 

 valves, and Univalves. The first order, 

 MULTIVALVE, is made up of Shells consisting 

 of more shelly parts or pieces than two. 

 Every part of a Shell which is connected 

 with a corresponding part by a cartilage, 

 ligament, hinge, or tooth, is called a valve of 

 such shell. The second order, BIVALVE, is 

 made up of Shells having two parts or valves, 

 generally connected by a cartilage or hinge ; 

 as in the Cockle and Mussel. The hinge is 

 entirely formed by the inner layer of shell, 

 and consists of either a simple cardinal pro- 

 cess, or a serrated edge, or of projections 

 called teeth, and corresponding cavities into 

 which they are inserted. To this hinge is 

 superadded a highly elastic ligament, com- 

 posed of a number of fibres parallel to each 

 other and perpendicular to the valves which 

 they connect ; which is a beautiful con- 

 trivance for the necessities of the animal ; 

 for by means of it, while undisturbed, the 

 valves are kept open, and the animal func- 

 tions are carried on without effort ; whereas, 

 if danger be apprehended, or any circum- 

 stances require it, the adductor muscles con- 

 tract, overcome the resistance of the hinge, 

 and shut the valves close until they may be 

 opened in safety. The valves of some bi- 

 valve Shells are formed exactly alike ; others 

 are very different ; one may be smooth, and 

 the other rough ; one flat, the other convex ; 

 and one is sometimes shorter than the other. 

 The Third Order, UNIVALVE, is made up 

 of Shells complete in one piece ; as in the 

 Periwinkle and the Whelk : and they are 

 subdivided into Shells with a regular spire, 

 and those without a spire. The shells com- 

 posing this order are far more numerous 

 than those of the two preceding, both in 

 genera and species. The spire is a prominent 

 feature of the Univalve ; and upon its being 

 lengthened or elevated, shortened or de- 

 pressed, &c., depends much of the generic 

 and specific definition. Shells increase in 

 size by the deposition of new layers inter- 

 nally upon those already formed. Each 

 new layer extends more or less beyond the 

 margins of the layer to which it is applied, 

 so that as the animal becomes older, its 

 Shell becomes larger and thicker. The outer 

 surface is generally covered by a thin layer 

 of membranous or horny matter named the 

 epidermis, and the inner surface is often 

 covered with a layer of a pearly nature. 

 It ; s universally found that the Marine 

 Shells of warm climates exceed all others in 

 beauty of colouring, and in taking a fine 

 polish. Several of the Land Shells also that 

 are met with in tropical countries are re- 

 markable for their bright colours and elegant 

 forms. River and Land Shells, with very 

 few exceptions, are thinner than those of the 



