89 
ROOM XIII.] NATURAL HISTORY. 
and Notarchus ; 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, ( Ckitonidce ,) 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- 
