74 
NATURAL HISTORY. [UPPER FLOOR. 
animals which have their branchiae placed on the right 
side 5 in a groove between the body and the foot, as the 
Pleurobranckus; and lastly, those in which the branchiae 
are placed along both sides on the inner edge of the man¬ 
tle, as the Patella , which has a simple conical shell, with 
its apex bent toward the head of the animal, and Chiton, 
which has the body covered by a hard cartilaginous shield, 
into which eight valves, laid one over the other, like plate 
armour, are inserted. 
Case 8 contains the Bivalve shells, the animals of which 
are compressed with a variously shaped foot, inclosed be¬ 
tween the two leaved mantles, having their two laminar bran¬ 
chiae 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, Cytlierea, Venus, and Venerupis, 
which have three teeth in each valve, and an external car¬ 
tilage ; Cyprina, Crassina, Pisidium and Cyclas, differ from 
the former, in having no syphons! inflection, bearing a 
thick periostracum; the last two are also only found in 
fresh water. Isocardia is peculiar for its strongly incurved 
umbones, 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 having an external and the 
latter 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 wedge-shells, from their shape, 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 P kolas, this skin is protected by one or more shelly 
plates. This caused the Linnean conchologists to consider 
the former as a univalve, having mistaken the tubes for 
the shell of the animal, and the latter for a multivalve 
shell. 
Case 12 contains the genera which have the lobes of the 
