ROOM XII.] 
NATURAL HISTORY. 
71 
The animals of the last two genera are so large, as scarcely 
to admit of their being withdrawn into their shells, and that 
of the last has a fleshy collar spread over the neck before 
the shell. Those which live in water have compressed con¬ 
tractile tentacula, the eyes of some, as in the genera Auri¬ 
cula or Carychium , being placed on the inner part, and of 
others, as in the genera Lymnea , Physa, Planorbis, and 
Ancylus, on the outer side of their base. The Clausilice and 
the three last-named genera have their whorls always turned 
to the left, and the Ancylus much resembles a Patella in 
shape, but has a notch in the muscular scar on the left side, 
where the hole is placed that leads to the lungs. Cycles - 
toma differs from all the other land mollusca, in having an 
operculum, and in the breathing cavity being Qpen in front. 
Case 8 contains the Bivalve shells, the animals of which 
are compressed, with a variously shaped foot inclosed be¬ 
tween the two leaved mantles, and have their two laminar 
branchiee placed on each side between the mantle and the 
foot. 
In some, the two leaves of the mantle are united toge¬ 
ther behind, and extended into more or less elongated tubes, 
as in the genera Artemis , Cytherea, Venus, and Venerupis ; 
these have three teeth in each valve, and an external car¬ 
tilage. Cyprina, Crassina, Pisidium and Cyclas, differ from 
the former, in having no syphonal inflection, bearing a 
thick periostracum ; the two last are only found in fresh 
water. Isocardia is peculiar for its strongly incurved um- 
bones, and very oblique teeth. 
Case 10 contains the genera Lucina and Loripes , which 
have an opaque white internal surface to their valves, and 
no syphonal scar ; the former has an external and the lat¬ 
ter an internal cartilage: also the Solens, Psammobia, and 
Tellina, which are elongated and gaping at one or both ends, 
the last having the hinder extremity obliquely twisted. 
Case 11 contains the genera Cardium and Donax, called 
also, from their shape, wedge-shells, which have only two 
teeth in each valve, forming a kind of cross. Then follow 
the bivalve shells which have no cartilage, the valves being 
separated from each other by means of muscles, placed 
over the umbones, and covered with a thin skin. In 
Teredo , this skin is simple, and the animals line the holes 
made by them with shelly matter, forming a tube; in the 
genus Pholas, this skin is protected by one or more shelly 
