104 
ROOM XIII. being provided with a hard shelly gizzard. The family of 
Nat. Hist. Sea Hares (. Aplysiadce ), so called from the form they as¬ 
sume when sitting on the rocks, have an elongated head 
and distinct tentacula, as the genera Aplysia , Dolabella, 
and Notai'dius; the latter has no shell. The species of the 
family of 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 branch iag 
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 Pleurobrancliidce , consisting of the genera 
Berthella and Pleurobranchus, which have a very thin 
membranaceous shell inclosed in the mantle: the fa¬ 
mily of the Umbrellidce , which includes only the genus 
Umbrella, has a suborbicular, flat, hard, external shell. 
This genus was formerly 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 
(Patellidxe ), 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, 
(Chitonidce ), as anomalous as the above, since instead of 
having a single shell, it has a row r of shelly valves, like 
plate armour, arranged in regular series down the mid¬ 
dle of the back; as in the genera Chiton, Acantliochetes, and 
Chitonellus: the second of these is peculiar for having a 
bundle of bristles placed on each side of the valves; and 
the 
