ICE 



ICH 



the stiff and pointed quills with which 

 they are armed, resembling those of the 

 hedge- Ao^, thougli usually larger. The 

 term porcupine is a corruption of the 



French porc-epin, or hog's-spine ; be- 

 sides, the general appearance and pecu- 

 liar grunt of the porcupine are not unlike 

 those of the hog. 



lA'NTHIN^. Oceanic shells ; a sub- 

 family of the Turhidcc, named from the 

 genus ianthina, comprising those fragile 

 violet and white snails which so much 

 resemble the Helicidce. The animal is 

 of peculiar structure. 



ICE, POLAR. The poles are pro- 

 bably surrounded by a continuous ex- 

 panse of ice, the borders of which have 

 been explored by navigators, and desig- 

 nated by various appellations, in reference 

 to the form and extent of its masses :— 



1. An Ice-berg, or ice-hill, is a vast 

 isolated mass of ice, sometimes rising to 

 the height of 100, and even 200, feet 

 above the surface, and extending beneath 

 it to a much greater depth. Ice-bergs 

 are formed by glaciers, which, some- 

 times terminating a precipitous edge on 

 the coast, and being gradually protruded, 

 at length break off, and fall into the 

 sea. 



2. A Field of Ice is a continued sheet 

 of ice, so large that its boundaries can- 

 not be seen from the mast-head of a 

 vessel ; it has sometimes an area of more 

 than 100 square yards, and rises above 

 the level of the sea from two to eight 

 feet. Ice-fields are generated in the 

 open sea, and are almost constantly 

 driving in summer, their general motion 

 being south-westward. They acquire 

 occasionally a rotatory motion, when 

 their circumference attains a velocity of 

 several miles an hour. 



3. A Pack of Ice consists of pieces of 

 ice broken off from a field, when driven 

 to the southward, and exposed to the 

 effects of a ground swell; these pieces 

 are about forty or fifty yards in diameter, 

 and extend so widely as not to be seen 

 over from the mast-head. 



4. A Patch of Ice is a collection of 

 pieces of ice, which assume a circular 

 form. When the collection is of an 

 elongated form, it is called a stream. 

 Pieces of very large dimensions, but 

 smaller than fields, are called floes. 

 Small pieces broken from larger masses 

 by attrition, are called brash-ice. 



5. A Hummock of Ice is a protube- 



173 



ranee raised upon any plane of ice above 

 the common level. A calf of ice is a 

 portion extending below the surface of 

 the water. 



ICE-SPAR. A sub-species of prism- 

 atic felspar, of vitreous lustre, translu- 

 cent and transparent, found at Monte 

 Somma near Naples. 



ICELAND-SPAR. One of the purest 

 varieties of calcareous spar, or crystal- 

 lized carbonate of lime. It is not, how- 

 ever, peculiar to Iceland. Of all crys- 

 tallized bodies, this exhibits most readily 

 the phenomenon of double refraction. 



ICHNEUMO'NID^. A family of the 

 entomophagous Terebrantia, named from 

 the typical genus ichneumon, and charac- 

 terized by their habit of depositing their 

 eggs in the bodies of other insects, on 

 which the young feed, when hatched. 



ICHTHYODO'RULITES (ix6luv, a 

 fish, 66pv, a spear, \i9o^, a stone). Fossil 

 spear-like projections from the back and 

 belly of a shark-like fish, supporting fins, 

 and serving probably also as weapons. 

 They occur in the carboniferous strata, 

 and seem to be identical with the bony 

 spine with which the Port Jackson shark 

 is provided. 



ICHTHYO'LOGY (ix^vp, a fish, \670f , 

 a description). That branch of zoology 

 which treats of Fishes, a class of animals 

 occupying the lowest station of the four 

 great divisions of the section Vertebrata. 

 They are distinguished by Cuvier, with 

 reference to their skeleton, into the 

 Osseous and the Cartilaginous. 



1. Osseous Fishes are those which have 

 a bony skeleton, the bony matter being 

 disposed in fibres; the sutures of the 

 cranium are distinct ; the maxillary and 

 the intermaxillary bones are, either one 

 or both, present. The sections are the 

 Pectinibranchii, the Plectognathi, and 

 the Lophobranchii. The first of these 

 comprises the orders Acanthopterygii, 

 Malacopterygii, and Apodes. 



2. Cartilaginous Fishes (chondropte- 

 rygii) are those which have a cartilagi- 



Inous skeleton, the bones being destitute 

 of fibres ; the sutures of the cranium are 

 13 



