JAN 



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thina. Mr. Lyell remarks " the janthina 

 fragilis has wandered into almost every 

 sea, both tropical and temperate. This 

 common oceanic shell derives its buoy- 

 ancy from an admirably contrived float, 

 which has enabled it not only to disperse 

 itself so universally, but to become an 

 active agent in disseminating other spe- 

 cies, which attach themselves, or their 

 ova, to its shell. Principles of Geology. 

 According to the account given by Bosc, 

 the janthina exhibits many remarkable pe- 

 culiarities. When the sea is calm, these 

 animals may be seen collected often in 

 large bands, swimming over the surface 

 by means of a floating apparatus consist- 

 ing of aerial vesicles produced by their 

 foot. During this action their head is 

 very prominent, and the foot is so ex- 

 tended that the float or line of vesicles 

 forms an angle with the middle of the 

 shell. When the sea is rough, the animal 

 absorbs the air from its vesicles, changes 

 the direction of its foot, contracts its body, 

 and lets itself sink. It does the same 

 when in danger from any enemy, and 

 like the cuttle-fish, has the power of 

 emitting a coloured fluid, which, by 

 darkening the surrounding water, serves 

 to conceal it from view. If the floating 

 apparatus be injured or destroyed, there 

 exists a reproductive power in the foot, 

 by which it can be restored. 



JA'RGON. The zircon jargon of Brong- 

 niart. A mineral, a variety of zircon. 



JA'SPER. (jaspe, Fr. pierre dure et opaque, 

 de la nature de V agate; jaspide, It.) 

 A sub-species, a variety of rhombohedral 

 quartz. It is an ingredient in the com- 

 position of many mountains. It occurs 

 usually in large amorphous masses, and 

 sometimes also crystallized in six-sided 

 prisms. Fracture conchoidal. It is said 

 to compose the substance of entire ranges 

 of the Asiatic mountains. When quartz 

 is combined with a considerable propor- 

 tion of iron and alumine, it loses its trans- 

 lucency and becomes jasper. There are 

 many varieties of jasper, distinguished 

 principally by their different colours, and 

 the arrangement of their colours. Mr. 

 Bake well states, " there can be little 

 doubt that jasper has been, in many in- 

 stances, formed by subterranean heat, 

 acting with great intensity on beds of ar- 

 gillaceous shale, containing iron." 



JASPI'DEAN. ^ Resembling jasper ; contain- 



JASPI'DIOUS. $ ing jasper. 



ICE/BERG. (from ice and berg, Germ.) 

 A large mass of ice, met with in cold re- 

 gions, floating upon the sea, sometimes of 

 enormous magnitude and great height. 

 Icebergs have been seen of the great height 

 of 300 feet, and as it has been ascertained 

 that for every foot above the surface of 



the sea-water there are eight feet below, 

 the whole thickness must be immense. In 

 a geological point of view, icebergs are to 

 be viewed as very important and power- 

 ful agents, inasmuch as they are the 

 means of transporting to great distances, 

 animals, plants, and rocks. 



ICHTHYODO'RULITE. The fossil dorsal 

 spine of certain fishes, armed with tooth- 

 like hooks, or prickles. These were long 

 supposed, says Prof. Buckland, to be 

 jaws, and true teeth ; more recently they 

 have been ascertained to be dorsal spines 

 of fishes, and, from their supposed defen- 

 sive office, have been named ichthyodo- 

 rulites, from the Greek words ixOi'c, a 

 fish, dopv, a spear, and Ai'0of , a stone. 



I'CHTHYOUTE. (from ixvQ, and Xi'0oe, 

 Gr. ichtyolite, Fr.) Fossil fish; a pe- 

 trified fish. Fossil fishes occur in all the 

 English formations, from the old red 

 sandstone to the tertiary deposits inclu- 

 sive. 



ICHTHYOLO'GICAL. Relating to ichthyo- 

 logy* or that branch of zoology which 

 treats of the structure, habits, &c. of 

 fishes. 



ICHTHYO'LOGIST. One who pursues the 

 study of ichthyology. 



ICHTHYOLOGY, (from i%0vc, a fish, and 

 Xoyoc, Gr. discourse ; ichtyologie, Fr. 

 ictologia, It.) That branch of zoology 

 which treats of the structure, classifica- 

 tion, habits, and history of fishes. 



ICHTHYO'PHAGOUS. (from ty^c, a fish, 

 and 0a'yw, Gr. to eat ; ichthyophage, Fr. 

 colui che non si ciba d" 1 altro fuorche di 

 pesci, It.) Feeding on fish. 



ICHTHYO'PHAGY. The practice of feed- 

 ing on fish. 



ICHTHYOSAU'RUS. (from t-x^vg, a fish, 

 and iravpoG, Gr. a lizard ) A fish-like 

 lizard ; an immense fossil marine -saurian 

 or reptile, having an intermediate orga- 

 nization between that of a lizard and a 

 fish. The name appears to have been given 

 to it by Mr. Konig. The genus comprises 

 many species ; some of these attain a 

 magnitude not inferior to that of young 

 whales. The head of the ichthyosaurus 

 resembled that of a dolphin, its teeth were 

 conical, sharp, and striated, and exceed- 

 ingly numerous, in some cases amounting 

 to nearly two hundred, not enclosed in 

 separate sockets, but as in the crocodile, 

 ranged in one continuous groove, or fur- 

 row, of the maxillary bone ; as also in 

 the crocodile, abundant provision was 

 made for replacing the old teeth, as they 

 were lost, by a supply of new ones. The 

 eye was of enormous magnitude, the 

 orbit in some instances measuring four- 

 teen inches in its longer diameter, and 

 Professor Buckland states, " We have 

 evidence that it possessed both micro- 



