The Fish World at Home. 227 



the first step in the formation of teeth is the simple production 

 of a soft vascular papilla, or pimple, from the free surface of 

 the membrane of the jaw, near the mouth ; but in the sharks 

 and rays these papillas do not proceed to sink into the sub- 

 stance of the gum, but become covered by caps of an opposite 

 .free fold of this membrane. These caps do not contract any 

 organic connection with the papilliform matrix (and in the 

 torpedo they are very loose), but as this is converted into the 

 dental tissue, the tooth is gradually withdrawn (the points of 

 the teeth at first lying flat downward, or in the direction toward 

 the mouth) from the extraneous protecting cap, and as they 

 become hard from being clothed with an enamelled surface, 

 they assume the upright posture on the border of the jaw." 

 The several rows of teeth are successively carried forward " by 

 action in the membrane itself on which they rest, until being 

 commonly broken or worn down by the violence to which they 

 have been exposed, by the time they have reached the outer 

 jaw, an exfoliation of the membrane itself has taken place, and 

 they drop off by a natural process of exfoliation, to be succeeded 

 by others, which are in their turn formed at the border of the 

 jaw nearest the mouth, and pass upward and outward." 



The greater number of sharks hatch their eggs in their own 

 bodies, but the ground sharks deposit their ova in singular 

 leathery bags, with long twisted strings at their corners, similar 

 to those of the skates, which are common on every beach. The 

 British sharks, which are most generally seen by sea- side 

 visitors, are dog-fish of small dimensions: but many larger 

 species frequent our shores, and Mr. Couch describes no less 

 than seventeen which belong to us, as more or less regular 

 inhabitants, and among them we find several quite worthy of 

 their formidable fame. One magnificent species, and less 

 ferocious than most of his brethren, performs a regular migration 

 along the west coast of Ireland, and the western islands of Scot- 

 land, and is the profitable subject of periodical attack. This is 

 the ' ' basking shark," or sun-fish, so called from its habit of 

 basking in the sunshine during calm, bright weather. It is 

 killed with a harpoon, and valued on account of its liver, which 

 weighs " two tons," and yields a large quantity of fine oil. 

 Mr. Couch says this is the largest of all true fishes, and he 

 gives a drawing of one captured in Cornwall, which measured 

 thirty-one feet eight inches in length. The two most extraor- 

 dinary-looking sharks are the " Thrasher," which is not " un- 

 common on the western and southern coasts of Britain in the 

 summer," and the ' ' Hammer-head," which is " a rare wanderer 

 in our seas." The first is eleven or twelve feet long, and 

 remarkable for the great length and strength of the upper 

 division of its tail, while the second has a head extended 



