SHE 



[ 402 ] 8 H 



Squaloids, the teeth are smooth on 

 the outer side, and plicated on the in- 

 ner ; sometimes the edge is serrated. 

 SHELL, (schah, Germ.) The hard 

 covering of anything ; the covering 

 of a testaceous or crustaceous ani- 

 mal. The crustaceous coverings ef 

 animals, as of echini, crabs, lobsters, 

 cray-fish, &c., are composed of the 

 same ingredients as bones ; but the 

 proportion of carbonate of lime far 

 exceeds that of phosphate of lime 

 in shells. 



The process employed by nature 

 for the formation and enlargement 

 of the shells of the mollusca was 

 very imperfectly understood 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 materials consti- 

 tuting each successive layer are 

 supplied by the organized fleshy 

 substance called the mantle, and 

 not by any vessels belonging to the 

 shell itself. The connexion be- 

 tween the animal and the shell 

 may be regarded as mechanical 

 rather than vital; for whatever 

 portion of vitality it might have 

 possessed when first deposited, all 

 trace of that property soon dis- 

 appears. 



Shells are found fossil in the most 

 ancient strata of the transition 

 period that contain any traces of 

 organic life, and many of these 

 agree so closely with the existing 

 species, that we infer their func- 

 tions to have been the same, and 

 that they were inhabited by ani- 

 mals of form and habits similar to 

 those which fabricate the living 

 shells most nearly resembling them. 

 The most prolific source of organic 

 remains has been the accumulation 

 of the shelly coverings of animals 

 which occupied the bottom of the 

 sea during a long period of conse- 



cutive generations. A large pro- 

 portion of the entire substance of 

 many strata is composed of myriads 

 of these shells, reduced to a commi- 

 nuted state by the long-continued 

 movements of water. Minute ex- 

 amination discloses occasionally 

 prodigious accumulations of micro- 

 scopic shells, no less surprising by 

 their abundance, than their extreme 

 minuteness; the mode in which 

 they are sometimes crowded to- 

 gether, may be estimated from the 

 fact, that Soldani collected from 

 less than an ounce and a half of 

 stone, found in the hills of Tus- 

 cany, ten thousand four hundred 

 and fifty-four microscopic cham- 

 bered shells. Of several species of 

 these shells, four or five hundred 

 weigh but a single grain ; of one 

 species, a thousand individuals 

 would scarcely weigh one grain; 

 and great numbers of them will 

 pass through a paper in which 

 holes have been pricked with a 

 needle of the smallest size. 



SHELL MAEL. A deposit of calcareous 

 earth and clay containing shells. 



SHI'NGLE. fscJiindel, Germ.) The 

 loose, water-worn, pebbles on the 

 sea- shore. 



SHOKLA'CEOUS. A name given by Dr. 

 Boase to certain primary rocks 

 which contain shorl; containing 

 shorl. 



SHORL BOCK. A genus of primary 

 rocks. Dr. Boase says, " Shorl- 

 rock is composed of quartz and 

 shorl, with the occasional addition 

 of a third mineral. Its species 

 generally possess the structure of 

 granite: but sometimes the con- 

 stituents are so intimately blended 

 that they cannot be distinguished, 

 forming homogeneous m asses which, 

 however, are so well characterized, 

 that they cannot easily be mistaken 

 for any other rock. Shorl-rock 

 occurs in distinct beds, but more 

 commonly in immense veins; in 

 both forms it is found in granite 



