S H A 



[ 234 ] 



SHE 



and containing the remains of myriads of | 

 shells, polyparia, &c. 



SHARK. (from icapxapiaQ, Gr.) The 

 squalus of Linnaeus. A genus of fishes 

 belonging to the family Selachii, order 

 Chondropterygii Branchiis Fixis. The 

 shark is a phosphoric fish. That tribe of 

 sharks, called by the French Regains, 

 which is thought to be synonymous with 

 the carcharias of the Greeks, and one of 

 which was probably the monster that 

 swallowed Jonah, are stated to exceed 

 thirty feet in length. The genus of sharks 

 may be considered as one of the most 

 universally diffused, and most voracious 

 of modern fishes. Several rows of teeth 

 are lodged in each jaw, but one only of 

 these rows projects, and is in use at the 

 same time ; the rest lying flat, but ready 

 to rise in order to replace those that have 

 been broken off, or worn down. The 

 shark is oviparous or ovo-viviparous, ac- 

 cording to circumstances. The vertebral 

 column is prolonged into the upper lobe 

 of the tail, and the tail is of great service 

 in enabling the shark to turn its body 

 so as to bring the mouth, which is placed 

 downwards beneath the head, into con- 

 tact with its prey. Sharks appear to 

 have existed throughout every period of 

 geological history. M. Agassiz has se- 

 parated the sharks into three sub-fami- 

 lies, each containing forms peculiar to 

 certain geological epochs, and which 

 change simultaneously with the other great 

 changes in fossil remains. The first of 

 these sub-families, or the Cestracionts, 

 commences with the transition strata, ap- 

 pearing in every subsequent formation 

 till the commencement of the tertiary. 

 Of the Cestracionts, one representative 

 only now remains, namely, the Cestra- 

 cion Philippi, or Port Jackson shark. 

 The Cestracionts possessed large poly- 

 gonal, obtuse, enamelled teeth, covering 

 the interior of the mouth with a kind of 

 tesselated pavement. The second sub- 

 family, or Hybodonts, commenced with 

 the muschel-kalk, is found throughout 

 the whole of the oolitic deposits, and dis- 

 appears at the commencement of the 

 cretaceous group. The teeth of the Hy- 

 bodonts were intermediate between the 

 blunt polygonal teeth of the Cestracionts 

 and the sharp-edged cutting teeth of the 

 Squaloids. The third sub-family, termed 

 Squaloids, appeared at the commence- 

 ment of the chalk deposits, and conti- 

 nues downwards to the present period. 

 In the Squaloids, the teeth are smooth 

 on the outer side, and plicated on the 

 inner ; sometimes the edge is serrated. 



SHELL, (schale, Germ.) The hard cover- 

 ing of anything ; the covering of a testa- 

 ceous or crustaceous animal. The crus- 



taceous coverings of animals, as of echini, 

 crabs, lobsters, cray-fish, &c., are com- 

 posed of the game ingredients as bones ; 

 but the proportion of carbonate of lime 

 far exceeds that of the phosphate in 

 shells. The following are the propor- 

 tions contained in the" shell of the lob- 

 ster : Carbonate of lime 60'0, phosphate 

 of lime 14-0, cartilage 26-0. The shells 

 of marine animals may be divided into 

 two classes, namely, porcellaneous and 

 mother-of-pearl, or membranous, shells ; 

 the porcellaneous shells have the appear- 

 ance of porcelain ; their surface is ena- 

 melled, and their texture often slightly 

 fibrous : the mother-of-pearl, or mem- 

 branous shells, are covered with a strong 

 epidermis, below which lies the shelly 

 matter in layers. Porcellaneous shells 

 are composed of carbonate of lime, ce- 

 mented together by a small portion of 

 gelatine ; most of the univalve shells, and 

 many of the convoluted beautiful shells 

 of the tropics, belong to this division. 

 Mother-of-pearl, or membranous shells, 

 are composed of alternate layers of car- 

 bonate of lime and a thin membranaceous 

 substance, which resembles coagulated 

 albumen in its properties. This mem- 

 brane still retains the figure of the shell 

 after all the carbonate of lime has been 

 separated by acids. Many membranous, 

 or mother-of-pearl shells, exhibit, on se- 

 veral parts of their internal surface, a 

 glistening, silvery, or iridescent appear- 

 ance ; to this structure the term nacreous 

 has been applied. 



This appearance is caused by the pecu- 

 liar thinness, transparency, and regularity 

 of arrangement of the outer layers of the 

 membrane, which enter into the formation 

 of that part of the surface of the shell. The 

 surface, which has thus acquired a pearly 

 lustre, was formerly believed to be a pe- 

 culiar substance, and was termed mother- 

 of-pearl, from a supposition that of it 

 pearls were formed. Assuredly, pearls 

 are composed of the same materials, and 

 have the same laminated structure as the 

 membranous shells ; but Sir D. Brewster 

 has satisfactorily demonstrated that the 

 iridescent colours exhibited by these sur- 

 faces are wholly the effect of the parallel 

 grooves, consequent upon the regularity of 

 arrangement in the successive deposites of 

 shell. 



The process employed by nature for the 

 formation and enlargement of the shells 

 of the mollusca was very imperfectly un- 

 derstood prior to the investigations of 

 Reaumur. His experimental enquiries 

 have established these two general facts ; 

 first, that the growth of a shell is simply 

 the result of successive additions made to 

 its surface, and, secondly, that the ma- 



