136 UNIVALVES. — HALIOTIS. 



it often becomes a matter of difficulty to distinguish the 

 one from the other. The form of all the Haliotides re- 

 sembles the human ear, excepting one, which is called the 

 H. asinina, or ass's ear, on account of its being much more 

 elongated or distended than any of the other species. 



There are three reasons which operate to create diffi- 

 culty in the arrangement of the species of this genus : — 

 First, the outside of the shell is generally loaded with ma- 

 rine substances, or else is so much decayed or worn as 

 not to offer a lineament of the original texture, thereby 

 precluding all possibility of judging by the work or colour 

 to what species it appertains. Secondly, as the interior 

 of the Haliotides is enamelled with a magnificent surface 

 of iridescent pearl, no great distinction can be made by a 

 reference to that part of the shell. Thirdly, as the beau- 

 ty of the shell is considerably increased by being wholly 

 or partially uncoated and polished, it is customary to sub- 

 mit it to some such beautifying operation ; which, how- 

 ever, with the surface at once removes all clue to the at- 

 tainment of the specific character. 



The exterior of the shell is generally composed of rugae 

 or tuberculations, over which pass approximate elevated 

 striae. In some species, foliations supply the place of tu- 

 berculations, as in the H. Midae or Midas' ear; the out- 

 side of which is wrinkled, and of a dusky white hue; but 

 the inside is most beautifully iridescent. It is often eight 

 or nine inches long. 



The back of almost the whole of the Haliotides is fur- 

 nished with a row of orifices near the margin ; varying in 

 number from eight to thirty-eight; of these from three 

 to seven are generally open, and the rest perfectly closed. 

 There are, however, three exceptions to this general char- 

 acter; for the H. imperforata, H. impertusa, and H. du- 

 bia are entirely void of any orifices. The former, which 



