3366 



THE MOLLUSKS OR SHELLFISH 



species (P. pullus) reaches the British shores* The Trochida have a wider range, 

 are far more numerous, and occur everywhere from the Arctic to the Antarctic cir- 

 cles. The beauty of sculpture and coloration of many of the species of the typical 

 genus Trochus is beyond description, and can only be appreciated by an examination 

 of the shells themselves. Most forms are littoral, or inhabitants of shallow water, 

 but a few, and these among the most beautiful, have been dredged at enormous 

 depths. Nearly twenty different species occur around the British coast, and some 

 of them are extremely elegant in form and structure. The family Delphinulid<z in 

 many respects resembles the Trochidtz, and is represented only by the single tropical 

 genus Delphinula. The ormers (Haliotidce], although few in number, form an im- 

 portant family, on account of the economic value of the beautiful pearly shells 

 which are employed in inlaying. They are ear shaped, depressed, with a very 

 smalt spire, and the last whorl, which practically contains the entire animal, corre- 

 spondingly large, and pierced with a series of holes commencing at the spire and ex- 

 tending to the front margin, in a line subparallel with the left side. The outer 

 surface is calcareous, and often beautifully sculptured and colored, but the interior 

 is lined with the most brilliant pearl. The holes in the shell admit water to the 



gills through a slit in the mantle 

 beneath, and on the edges of the 

 slit there are three tentacular proc- 

 esses which the animal passes 

 through certain perforations. Hal- 

 iotis, like the limpets, has a large 

 foot, adapted for adhering to rocks. 

 Some of the larger forms hold on with 

 such tenacity, that it is impossible to 

 remove them without damaging the 

 shells, except with the aid of hot 

 water. The genus is most abundantly 

 DOLPHIN SHELL, Delphinula ladniata. represented in Japan, California, and 



(Natural size.) Australia; but is also met with in 



New Zealand, the Philippine islands, 



Oceania, some parts of the Indian Ocean, and round the African coast, while one 

 species (H. tuberculata) ranges as far north as the Channel islands. They are lo- 

 cally known as ormers, and are consumed in considerable quantities. When properly 

 prepared and cooked, they make a savory dish. It is worthy of notice that Hal- 

 iotis does not inhabit the South-American coasts, nor is it found on the Atlantic side 

 of North America. Until some fifty years ago, the typical genus of the family 

 Pleurotomariidce was supposed to be extinct. Four recent forms are now known, 

 one from Japan, one Moluccan, and two West Indian. Two of these are larger 

 than any of the extinct species. Pleurotomaria has a trochiform, discoid, or globose 

 shell, pearly within, with a more or less deep slit in the outer lip of the aperture; 

 the latter being the characteristic feature of the group. It has existed since the 

 Paleozoic epoch. In Trochotoma, an allied genus, there is a hole behind the lip in 

 place of the slit, and Polytremaria exhibits a whole series of perforations. The 



