GASTROPODS. 
365 
about high-water mark. The shells are distinguishable by a groove on the right 
side of the inner surface, interrupting the muscle by which the animal is attached. 
Siphonaria much resembles Auricula as regards the pulmonary cavity, which is 
adapted for breathing air or water, there being both a lung and a modified gill. It 
is, however, protected or closed by a valve-like lobe of the mantle. The horny 
jaw has a smooth and curved cutting edge. The radula consists of a single central 
or rachidian tooth and numerous laterals, very similar in type to that of certain 
groups of Pulmonata. About fifty species are known. The genus has an extensive 
range from Vancouver Island in the north to Cape Horn in the south. One species 
occurs on the coast of Portugal, but the majority inhabit eastern seas. In Gadinia 
the breathing-chamber is without any sign of branchiae. It is a limpet-like animal 
with a circular foot by which it attaches itself to rocks or stones, in the same 
manner as limpets. It is probably a poor traveller. The pulmonary orifice is 
placed on the right side near the head, and is closed by a small valve. The head 
is large, without tentacles, with the sessile eyes placed at the sides. Mr. Dali, who 
watched a colony of G. reticulata, noticed that “ as long as the rock on which they 
were remained damp, they continued with the margin of the shell firmly applied 
to it. As soon as the boulder became dry, under the hot sun, I perceived a simul¬ 
taneous motion in the colony. Each shell was raised above the surface of the 
stone, the head and foot were protruded, and the orifice of the pulmonary cavity 
was expanded. They were evidently enjoying the warm air.” 
Suborder Pteropoda. 
Until within the last few years, the molluscs included in this suborder were 
considered to constitute a separate class. Some systematists located the group 
near the Cephalopoda, others, believing them to be of a more degraded type than 
the Gastropoda, assigned them a position between that class and the bivalved 
Mollusca. Dr. Morch and others, long ago' pointed out the affinity of the 
Pteropods with the Opisthobranchs, and Dr. Pelseneer has come to the con¬ 
clusion that these molluscs should be included in two of the Tectibranchiate 
groups of the Opisthobranchs. The Pteropods seldom come near land, except 
when driven by currents and tempests, and although they rise to the surface 
principally at night, they have been observed during the daytime. They are 
characterised by two delicate fins or lateral expansions of the foot ( parapodia ), 
placed at the anterior end of the animal, and used in swimming, being moved with 
considerable rapidity, like the wings of a butterfly. When they wish to sink, 
their fins are contracted, and the anterior part of the body, in some cases, is more 
or less withdrawn into the shell or the mantle, and they thus fall to the depth 
desired. At times they remain stationary, by keeping the fins merely extended. 
Some— Clio, Pneumoderma, for example—adhere to floating bodies by means of 
oral appendages provided with suckers, others by means of a sufficiently developed 
foot. The fins, termed epipodia by Huxley, are sometimes united, forming a single 
disc, sometimes partly connected, or, finally, entirely separated. The Pteropods 
are hermaphrodite, or have the sexes united in each individual. They deposit their 
ova in the form of long bands which float on the surface of the sea. They are 
