39 6 
MOLLUSCS, 
form and structure. The family Delphinulidce in many respects resembles the 
Trochidce, and is represented only by the single tropical genus Delphinula. The 
ormers (Haliotidce), although few in number, form an important 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 small spire, and the last whorl, which 
practically contains the entire animal, correspondingly large, and pierced with a 
series of holes commencing at the spire and extending to the front margin, in a 
line subparallel with the left side. The outer surface is calcareous, and often 
beautifully sculptured and coloured, 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 
processes 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 
represented in Japan, California, and 
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 locally known as ormers, and are consumed in considerable quantities. 
When properly prepared and cooked, they make a savoury dish. It is worthy 
of notice that Hcdiotis 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. 
Plcnrotomaria 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 Palaeozoic epoch. In 
TrocJcotoma, an allied genus, there is a hole behind the lip in place of the slit, and 
Polytremaria exhibits a whole series of perforations. The anatomy of Pleuroto- 
maria shows that it has close relationship with Hcdiotis. Like Trochus, it is 
furnished with a horny spiral operculum. A magnificent specimen of P. 
adamsoniana is exhibited in the shell-gallery at the Natural History Museum. 
Scissurella is another genus of tins family, containing a number of very minute 
shells with a slit in the lip like Pleurotomaria ; S. crispata being a pretty 
little mollusc found in Britain. The members of the so-called key-hole limpets 
(Fissurellidce) mostly have shells like those of the limpets in form, but either 
perforated at the apex, as in Fissurella, slit at the front margin ( Emarc/inida ), or 
dolphin-shell, Delphinula laciniata (nat. size). 
