<!] VRKS AMI RAY-FISHES. 7 



or at the approach of danger; but in the generality of Sharks, 

 if not in all, the exposed surface becomes oval by means of a 

 structure of the skin above and below the globe, which, when the 

 fish finds itself hooked on the line, or othenvisc in danger, is made 

 to close over it, somewhat after the manner of the eyelids in 

 birds. The globe itself is supplied with muscles to direct its 

 actions, and its sphere of motion is still more extended by means 

 of a contrivance, in which we see a remarkable instance of the 

 skill by which what might appear to be a new organ, is produced 

 by the simple lengthening out of a well-known part into a new 

 form and for a new but important use. On examining the cavity 

 in which the eye of the Shark revolves, we find that the globe, 

 which is the immediate seat of the power of vision, is lifted from 

 the bottom, on which, in other animals besides those of this great 

 family, it rolls, and is placed on a small table that itself forms 

 the top of a slender pillar, the bottom of which is fixed on the 

 bony circle of the common ocular cavity; or more properly 

 speaking the pillar itself, which leans a little forward that it may 

 be accommodated to the most usual direction in which objects are 

 viewed, is an extension or modification of the orbitary process 

 of what anatomists term the sphenoid bone. The height of this 

 ocular pillar has the additional advantage of allowing a greater 

 length to the muscles which move the eye, and by so doing, of 

 providing for a more sudden, as well as a more extensive action 

 of the eyes in prowling for their prey. 



The remarkable nature and arrangement of the teeth in Sharks 

 and Rays has long attracted the attention of naturalists, and 

 great have been the mistakes which have been committed in 

 describing them and the process by which they are produced, 

 and also the succession of them continued. In other orders of 

 fish they take their rise from a membrane which clothes the 

 jaws: for we exclude the consideration of those which are found 

 in the gullet, tongue, and palate and they rise immediately from 

 this foundation in an upright direction. Here they remain at- 

 tached for a time, and then they fall away, at uncertain intervals, 

 to be succeeded by others. With the Sharks and Rays it is 

 widely different; and the particulars of the process by which 

 they are produced have been first and most extensively described 

 by Mr. Owen; but they had to some extent been observed by 

 myself, before I had obtained an opportunity of learning them 

 from that gentleman's more extensive discoveries. 



