89 
ROOM XIII.] NATURAL HISTORY. 
and Notarclius ; the latter has no shell. The Syphonarice 
have an external conical shell* with a groove in the side 
for the passage of the water to the gills* as those of the 
genus Syphonaria . The genus Gadinia appears to be 
very nearly allied to the preceding; but the grooves are 
placed in the front of the muscular scar* whereas in the 
former they are placed in the side of it. 
The other animals of this order have their branchiae 
placed on the right side of the body* in the groove between 
the edge of the mantle and the foot, as in the family of the 
Pleurobranchidce * consisting of the genera Berthella and 
Pleurobranchus * which have a very thin membranaceous 
shell inclosed in the mantle: the family of the Umbrellidce , 
which includes only the genus Umbrella* has a suborbi- 
cular* flat* hard* external shell. This genus was for¬ 
merly supposed to have its shell placed on its foot* and 
was called Gastroplax * but this has been proved to be an 
error. 
Case 50 contains the fourth order of Gasteropodous 
Mollusca* which have their gills in the form of plates on 
the back* or placed in a series round the edge of the mantle 
of the animal. It is only necessary here to refer to those 
which have the gills of the latter form* as they alone are 
provided with shells; as the family of Limpets* ( Patellidce *) 
which have a simple conical shell* with the apex directed 
towards the head of the animal* contrary to what prevails 
in almost all other shells : it consists of the genus Patella : 
also the family of Sea Woodlice* ( Chiioriidce ,) as anomalous 
as the above* since instead of having a single shell, it has 
a row of shelly valves* like plate armour* arranged in re¬ 
gular series down the middle of the back ; as in the genera 
Chiton , Acanthochetes , and Chitonellus: the second of 
these is peculiar for having a bundle of bristles placed on 
each side of the valves ; and the last* for the valves being 
nearly hid in the mantle of the animals. 
Cases 53—60 contain the fifth class of Gasteropodous 
Mollusca: they respire free air, which is received into a 
cavity between the mantle and the back* lined internally 
with numerous reticulated vessels. They are mostly ter¬ 
restrial, and when aquatic* they come to the surface of 
the water to respire; but they have the power of sus¬ 
pending their respiration* for a considerable time dur- 
